Thank you Amigakit!
I ordered my 4-device IDE interface at lunchtime yesterday and it arrived in the post this morning.
I've set it up with the HD and CD-ROM on the first channel and the SD to IDE adapter on the second and it all seems to be working brilliantly. I'm just copying from the CD to the SD adapter to test sustained load and it's done 200MB so far without crashing.
If anyone else is thinking of doing what I have, and using this with an A1200, I'd recommend it but don't go too long on the IDE cables. I have 25cm ones from the board to the IDE devices, and Amigakit 2.5 inch to 2 x 3.5" IDE between the Amiga and the interface (with the interface on the first connector). Any longer cables and I'd get random crashes.