Those little screws (called ‘grub screws’) that hold on the handles of the internal doors sometimes work loose and fall out. It’s very annoying, and replacements are impossible to find in the shops.
However, one owner has found them online:
The type needed is M6 thread, 6mm grub screws with a cone point, which need a 3mm allen key* to screw in. DoorFurnitureDirect.co.uk
Thanks for the tip, Simon!
* Allen keys (picture) are those hexagonal tool you can get from Deards or almost any cheap goods shop.