Hi gregers,
Thanks for your quick response but just to clarify, I only took the key out while I was taking the photos to show it with and without. However, the key was always in the lock during my attempts to remove the cylinder and of course to ensure the cam wasn't offset.
The good news...
Long story short, five days ago, my missus lost her keys so as a precautionary measure I decided to replace the locks on both the front and back doors. They are uPVC doors with Yale euro double cylinder locks. I removed my old cylinders to measure them to make sure I ordered the correct sizes...