Thanks for the reply.
The concept I've been working on has undergone a major revision - wow, that sounds like I've really done something great!
I've realised that the way I want it to work will only work if I have multi-site - and you guessed it - I have installed my current site in its own subdirectory - because I used named virtual hosts and it seemed a good idea at the time.
So, I'm going to have to reinstall in the root directory and start from scratch. Such is life.
I'm sure I'll be back, though as I really like the Syntax theme.
Thanks again for your help