Well, I've managed to get basic reading working, or rather the basic circuit and reading using the sample program. However, thats half the battle. I was struggling a little due to lack of proper chips, but I've managed to pick up the correct ones, so I'm back on track. I'll get the core Init/Read/Dir working, then I'll switch to the Plus/4 and see how I get on from there.
I suspect that I'll do it as an expansion card and not USER PORT plug-in though, as it allows me to download via my cable to program it - and perhaps even use my remote debugger which is partly written.