Well, finally the wiring was completed last week. There were a few teething problems, the main one being that I had forgotten to cut the copperclad on the sleepers at the baseboard edge! Other than that a point motor going the wrong direction, and a couple of forgotten wires, were easily fixed!
At the baseboard joint I didn't want to have to have a plug and socket - apart from the extra hassle there simply isn't space, the way the main board slides snugly into the alcove. So I soldered to the ends of the final extra-wide sleeper a couple of brass squares, the ends folded down and the inner corners folded up to a Vee. On the fiddle yard I soldered brass wires to the copperclad sleepers to make contact with the brass plates when the boards come together, the way the board slots into place makes this work surprisingly well.
Also on the fiddle-yard I soldered brass sheet strips to the cassette-end of the lead track, this makes contact with the copperclad strips accross the ends of the cassettes.
So all works well? No, not quite. Test running has shown that while the Microtrains couplings flick over as they pass over the burried magnets, they don't actually uncouple when coupled up! I've used these magnets under the track fine on Pen-Y-Bryn Quarry, but not here. I'm not sure why, perhaps the granite ballast is too dense? I've chipped some away but little improvement. Maybe the Homebase magnets are weaker than the B&Q ones? I don't know how to address this problem yet ...