Bristol has a pretty good startup scene (I've recently moved here). Have you checked out some of the local networking and support groups? There is some stuff concentrated around Engine Shed http://www.engine-shed.co.uk/

If you have finances you could always employ someone in a CTO type role rather than take on a co-founder.

If I were in your position I think I would define what I actually need of this additional senior person. Consider what skills they should have, what input you want of them. Then start chatting to people - if you find someone who seems ideal then you can can find out what kind of terms they would be willing to come on board under.