Simon,
Welcome, I to have the CL500M with most of the extra's, I've had mine for fifteen years and have no reason to improve on it it does everything I require of it.
I'm not that far from you, I live near Bedford and Sandy.
The only part of the lathe I dont use much is the Mill attachement only because I have two small milling machines that I use.
I have one mod that I want to do and that is fit an inverter with a three phase motor otherwise I'm happy with the lathe.
As to projects pick something that will be of use to you, I make parts for my Stationary engines and home maintenance, as said the Workshop Practice books are very good.
Martin P