Forgot to update this last night...
Yesterday's main tasks included:
1. Un-seize the heater box.
The demister side of things was already moving freely, but the cabin heater side not so much. After liberal application of WD40 (I've run out of Plusgas) and some careful violence I managed to get it moving nicely. The cable needs the termination sorted out as the sheath can move so it doesn't close the flap fully without being poked, but that's a job for the snagging list. At least it's not stuck fully on for now.
2. Reconnect the gear linkage.
Having replaced the bit in the car with a freely moving one from the spares stash, it needed to be actually attached to the gearbox. Easy enough job with the engine still being out as you can sit in the engine bay and just reach around the side of the box. I couldn't find the box of split pins to save my life, so the drawer full of random bolts, screws nuts and washers was raided and I just stuck a locknut on a bolt.
3. Trial attach the seat to the existing frame.
Very much a "proof of concept" effort with some large (actually slightly too large) exhaust clamps. Proves that it fits perfectly though, and to be honest it's absolutely rock solid - and far more substantially anchored than it was originally - by four tiny little (Imperial equivalent of M4) bolts through paper thin steel plate without even a washer to spread the load. Seatbelt is anchored to the chassis too, not the seat so it's not exactly going to see massive loads.
Also having the seat in place meant I could stick the seatbelt back in. This was a bit like trying to wrestle with an octopus while I untangled it and worked out which end went where, but got there in the end.
The seatbelt buckle and pretensioner assembly will be taken off the side of the seat shortly as they're no longer used.
Having a seat in place that doesn't fall apart when you sit on it and having a seatbelt there has made it feel a LOT more like an actual vehicle that might see the road again...