It appears my A1200 motherboard had the clockport header installed in the wrong place! The 22-pin header in P9B had been installed at the crystal end, and not the end closest to the trapdoor slot.
This is a known issue with some A1200 motherboards. Kudos to you for being able to solve it so easily, I know I'd be chewing all my fingernails off if I had to do this much soldering on an A1200 motherboard!
