The Derwent was not a rare engine, and many were saved for museums. Plus, I'm pretty sure
Welsh Pirate is going to use them to make light sabers since he posted his question in this forum. I'm willing to bet he isn't restoring a turbojet engine.
chibobber , it is possible that a bike shop may have a compatible spanner, but quite a few aircraft manufacturers use specialized tools for maintaining engines. The tools may only be good for that engine, or perhaps the only for that manufacturer. Even a different model of engine in the same line may need a different toolset. You could spend a lot of time going to bike different bike shops and not find what you are looking for.
Halliwax 's idea of putting it into a vice and using a drift punch is probably the safest idea. Especially if he uses soft jaws and a softer metal punch.
I don't see any corrosion on the part, so hopefully it will come apart with little effort.
TazMan2000