Continuing the saga for those interested or amused; the replacement instrument set arrived, got installed and failed to help.
I now know there's unlikely a problem with my instrument panel, the trouble is I'm now looking at the ECU and wondering what's going on there.
Looking at the wiring diagram in the service manual; both the speed sensor and the fuel pump/ indicator send wires directly to the instrument panel, but they also send one to the ECU. The gear indicator seems to go only to the ECU. I've no way of testing wires other than to confirm continuity, but the biggest problem at the instrument panel plug was the cabling can't be got at so I had no way of telling which pin was which. I've a feeling the ECU plugs will be similar.
I've yet to pull the ECU out of it's cosy little hole so that's job one this evening. As the issue is intermittant I'm still hoping it's a loose connection that's causing the problem, but if I still draw a blank after cleaning the plugs, then I'm tossing up whether to go to a garage for them to figure out or just shoving it in the canal.
On a positive note, after popping the clutch cover off (and spending way longer than planned removing the old gasket) I took up the slack between the plates and the clutch is feeling great again.