1. Diagnose before replacing parts

Open the window carefully and inspect the handle, locking strip, keeps, hinges and seals. Try to identify whether the problem is stiffness, poor closing pressure, a draught, a loose handle or visible wear.

• Check whether the handle turns freely when the window is open.
• Look for loose screws, missing keeps or a sash that has dropped.
• Clean dirt from tracks and locking points before assuming parts are broken.
• Take photos of part numbers and fixing positions before ordering replacements.

2. Clean, lubricate and check alignment

A surprising number of stiff windows improve after cleaning and light lubrication. Use a suitable silicone-based lubricant on moving metal parts, then test the handle gently. If the sash catches, check whether hinge adjustment or a worn friction stay is the likely cause.

• Do not use thick grease that collects dirt in the mechanism.
• Tighten loose fixings carefully without stripping the frame.
• Check that locking points meet the keeps without scraping hard.
• Stop if the handle still feels forced, because the gearbox may fail.

3. Replace tired draught seals like-for-like

If the window closes but feels draughty, inspect the rubber gasket. Pull a small section free and match the profile before buying replacement seal. Clean the channel, press the new seal in evenly and avoid stretching it as you fit.

• Match the seal profile rather than guessing from width alone.
• Work in short sections and keep corners neat.
• Avoid sealing drainage slots or blocking designed ventilation paths.
• If the glass unit has condensation inside, the sealed unit has failed and needs replacement.

4. Decide when hardware needs replacing

Loose handles, worn spindles, tired friction stays and failed gearboxes can often be replaced, but parts need to match. Measure carefully and support the sash if hinge work is involved.