Hi Ian
Sounds like you've bought a good bike there, one which will be fun to fettle rather than one which will be a frustrating money pit. I really like the smaller capacity British bikes like your C11, the aforementioned C12 I used to borrow, the Tiger Cub which was my first bike etc. There's something about them that I find easy and rewarding to ride and work on. I wonder how many thousands of blokes had a British 250 or below in the shed to go to work on and were happy to be one up from a bicycle.
Ian