After discussion here, bought a thumbturn lock for our uPVC front door. Ordered Tuesday, delivered yesterday (sorry, B&Q).
Fitted in 1 minute.
Slight downside is we need 3 more keys cut ... however looking at the old unit clearly in a moment of brain fart[1], I wondered if it was possible to swap barrels - in which case no new keys needed.
Inspected the unit and noticed a circlip which looked like it held the barrel in. Quick prise and ... loads of springs and tumblers. Somehow I think new keys are simpler.
[1]Of course, thinking about it the whole concept was flawed from the start, as it's not just the barrel that needs changing, but the tumblers in the casing.