Funny timing. As it happens now that I've basically finished the first phase of my 6 assessment (mechanicals and body) I've spent time looking at the airbag lights. The MOT will be booked soon and I'm f**king fed up of jiggling the wires under the seat and/or resetting the code with PP and simply hoping for the best. The lights inevitably come on again very soon afterwards...
From my undertanding, the airbag light can still relate to poor wiring under the seat.
I took both seats out and checked the wiring to the airbag ECU with a multimeter - all fine. The wiring on the seats has been re-made by me and previous owners and was a mess. I decided to cut out all the shoddy joins and use these automotive style connectors to re-make the connections well: https://www.ebay.co.uk/itm/2-3Pin-Way-Car-Auto-Waterproof-Electrical-Wire-Connectors-Plug-Socket-10-20-30pc/173975177146?ssPageName=STRK%3AMEBIDX%3AIT&var=472757536578&_trksid=p2060353.m1438.l2649
One thing I noticed was that the single earth wire on the driver's side was chafed through to the copper from rubbing on the steel frame of the seat. This may have been the source of the problems.
I installed the seats with the new connectors, neatly zip tied the wires to the seat base, used the original 3-connector mounting plastic with the new connectors to keep them from dangling in the wind, and turned on the ignition. No more lights, thank god. I provoked the system by moving and flipping both seats many times but they're still off and I didn't event need to get out PP.
I would advise doing a methodical, thorough job from beginning to end to ensure that, if you do still have lights on after this work you'll know that the under-seat wiring is no longer the cause.
The SRS on a 306 shouldn't have to be an utter bas**rd all the time; it isn't on most other cars. It's just taking the time to do a really neat and secure job whilst checking the entire exposed wire lengths for faults that fixes it I think.