All these doors use a 'Euro Cylinder' lock and can suffer being 'Lock Snapped' or, more rarely, 'Lock Bumped'.
When I get home tonight I'm measuring my doors and ordering a set of Avocet ABS Snap Secure locks and fitting them myself. The Avocets are the only ones that guard against this problem (as far as I can tell) and have all kinds of patents and accreditations that other manufacturers have not (or can't).
I'll let you do your own research into how the process works but it's eye-opening for sure

I've been on Avocet's website and it seems you Order the locks through approved distributors - they work out at about £50.00 each with a set of special keys.