Thanks to everyone for your advice and suggestions -- its really reassuring to hear that this is a difficult skill that most people struggle with -- gives me hope!
I've been following what you guys say: slowing it way way down and can see I am beginning to get it, slowly. It is bizarre and frustrating when your brain knows how to do it, but your hands seem to have a mind of their own!
One of you said 'have fun' at the end of your post, which I think is the bit I'm struggling with: it really is no fun at all playing a so slowly that you can't even hear the tune, and playing it over and over again for hours.
I wanted to ask whether this is something that you have to learn slowly in this way for every tune, or whether once you get it, its a skill that you can apply to any 12bar blues tune? To break up the monotony of swanee river, I worked out the different parts for the introduction to What'd I Say (Ray Charles), but again, same problem: putting the hands together is impossible. Will there come a time where I have developed this ability such that I can play any bass pattern in the left, and any melody in the right, without having to build it up bit by bit?
Thanks again for all your help.
JB