Thanks for the tips, Metro in Birmingham and Farnell's look reasonable price-wise. I forgot, I've also used Transpacolour in the past, but it was ages ago.

Tom: yes, colour printing at home really isn't going to be an option for me any time soon. Even once I do get an enlarger for 6x6 or even 6x9, I expect it'll be 99.99% black and white.