A uPVC door that is hard to lock almost always has a door problem, not a lock problem. Over time the door drops on its hinges, the locking hooks stop meeting the keeps on the frame, and the gearbox inside the door takes the strain every time you lift the handle. Caught early, the fix is usually an adjustment rather than a new lock.
This guide answers four questions people in the UK ask Google most about uPVC locks: why the door is hard to lock, how to tell whether you have anti snap locks, whether WD40 helps, and what a locksmith should charge. For a step by step fault finder, see our guide to uPVC door lock problems.
In short:
- If the door locks easily while open but not when shut, the lock works. The door is out of line, and a hinge or keep adjustment usually fixes it.
- Forcing a stiff handle is how a cheap adjustment becomes a gearbox replacement.
- An anti snap cylinder is marked TS007 with three stars, or Sold Secure Diamond. A cylinder with no marks should be treated as standard.
- WD40 is a cleaner, not a lock lubricant. Use a dry graphite or PTFE lubricant in the cylinder.
- Agree a fixed price before a locksmith sets off, and know you can refuse a different price at the door.
Why is my uPVC door so hard to lock?
The usual cause is misalignment. A uPVC door locks with a multipoint mechanism: lifting the handle drives hooks, rollers or bolts along the edge of the door into keeps on the frame, and turning the key holds them there. When the door sags, swells or the frame moves, those parts no longer line up, so lifting the handle means dragging metal past metal.
Common reasons a door drops out of line:
- Hinges settling or working loose, which lets the door sag at the lock side.
- Seasonal movement. uPVC expands in heat and contracts in cold, so a door that locks fine in spring can bind in summer.
- Glass settling in a glazed door. If the glazing packers shift, the panel no longer carries the door's weight evenly.
- A worn gearbox or mechanism, which makes the handle stiff even when everything lines up.
The door open test: which part is at fault?
Three checks, in this order, tell you what has failed before anyone quotes for parts.
| Check | Result | What it means |
|---|---|---|
| Open the door and lift the handle | The hooks come out smoothly | The mechanism works |
| Open the door and lift the handle | Stiff, or nothing moves | The mechanism or gearbox is worn or broken |
| With the door open, turn the key | It will not turn | The cylinder is at fault |
| Both work open, but not shut | Hard to lock when closed | The door is out of line with its keeps |
Stop forcing the handle once it starts to bind. The gearbox takes that force, and when it breaks the door can jam locked or refuse to lock at all. A hinge or keep adjustment is a small job. A door that will not open is an emergency.
How do I know if I have anti snap locks?
Look at the face of the cylinder, the part your key goes into, with the door open. An anti snap cylinder carries one of two markings:
- TS007 with three stars and a Kitemark.
- Sold Secure Diamond (SS312).
West Yorkshire Police name these as the two nationally recognised standards for cylinders, call three stars the minimum, and advise that cylinders should not be left protruding from the door or handle (West Yorkshire Police, euro cylinder advice, retrieved 19 October 2026). Three stars can also come from a one star cylinder paired with a two star handle, so check the handle too. If you see no marks at all, assume a standard cylinder. Our answer on how to check for anti snap locks has more detail.
Why snapping matters. Most uPVC and composite doors use a euro cylinder, and a standard one has a weak point in the middle, where its fixing screw passes through. A rated cylinder is designed so that an attack breaks off a sacrificial front section and leaves the locking part inside the door. Fit it so it sits no more than 3mm proud of the handle. We do not publish how snapping is done: the useful information for a homeowner is how to check and what to fit.
Is WD40 good for a stiff door lock?
Only as a short term fix. WD40 is a cleaner and water dispersant. It can free a sticky cylinder for a while, but the residue it leaves can attract dust into the pins, and the lock may end up stiffer than before.
Use the right product for each part:
- Inside the cylinder: a dry lubricant, graphite or PTFE. Apply a little, then work the key in and out several times.
- On the mechanism along the door edge: a light grease or oil on the hooks, rollers and bolts, then lift and lower the handle to spread it.
- Never cooking oil, which gums up.
Twice a year is enough. The weekends the clocks change make an easy reminder. If lubricating does not help, the cause is alignment or wear, and more lubricant will not fix it.
How much should a locksmith charge to change a lock?
The right price is the one agreed before the work starts. Our own prices are published, and our guide to locksmith cost in London compares published prices across the city.
The risk of overcharging is real. A BBC London investigation filmed a company trying to charge many times its original quote. One couple, both 85, had expected to pay under £50 after being locked out and were billed £2,802 (BBC London report via AOL, retrieved 19 October 2026).
Before anyone sets off, ask for:
- The company name and an address you can look up.
- A fixed price, not a "from" price.
- What the price includes: callout, labour, parts and VAT.
- An arrival time.
If the price changes on the doorstep, you can say no. A locksmith who wants to drill the lock before trying anything else should also make you pause. Our guide on how to spot a fake locksmith lists the other warning signs.
What drives the price of a uPVC lock repair:
- Adjustment or replacement. Realigning hinges and keeps is less work than replacing parts.
- Which part. A cylinder is the cheapest change. A gearbox can often be replaced on its own, which costs less than a full multipoint strip.
- Universal or exact match. A universal replacement mechanism is usually cheaper than a branded original.
- Time. Evening, weekend and night work costs more than a weekday visit.
Quick answers
Why do I have to lift my uPVC door handle to lock it? Lifting the handle throws the hooks and bolts into the frame, and the key then locks them in place. Lifting is normal. Having to lift hard, or lean on the door, means it has dropped out of line.
Why will my uPVC door key not turn? On most uPVC doors the key will not turn until the handle is fully lifted. If it still will not turn, the door may be out of line, the cylinder may need cleaning and dry lubricant, or the key may be worn. Do not force it: a snapped key turns a small job into a large one.
Can I replace a uPVC door lock myself? The cylinder, yes, if the door opens, you have a working key and you measure it correctly, from the centre fixing screw to each end. A full multipoint mechanism must match the backset, case size and hook positions exactly. See can I replace a door lock myself.
How do I measure a euro cylinder? Measure from the centre of the fixing screw hole to the outside end, then to the inside end. A cylinder measuring 35mm outside and 45mm inside is a 35/45. Always note which side faces outside.
Need it fixed?
If your door will not lock tonight, do not leave it. Send us a photo of the door edge and the handle, and you get a fixed price back before we set off. We cover uPVC door and window mechanisms across south east London.
Sources
- BBC London investigation into locksmith overcharging, syndicated by AOL: Secret filming catches locksmith charging 10 times quote, retrieved 19 October 2026.
- West Yorkshire Police: Euro cylinder advice, retrieved 19 October 2026.
