Update:
Thanks to everyone for their help and suggestions.
To summarise my problems were as follows
EAS,ABS,SRS,TC, Airbag messages came up on the dash but then not be there when restarting.
Sometimes the messages were accompanied by alternator fault. Sometimes the messages would be accompanied by the speedo not working. If the faults came up while driving they usually cleared within a mile. Everything was operating correctly and this intermittent behaviour lasted about 2-3 weeks.
Finally the fault went hard, the ABS pump stopped working, diagnostics either showed no faults or couldn't communicate with the ECU. And finally, after the fault had gone hard, the engine wouldn't start without a great deal of effort.
I sourced a reasonably priced second hand fuse box and despite the vehicle standing for a couple of weeks the old girl fired straight up, no faults dispalyed, everything working as it should.
Unfortunately it is an older fuse box than the one it replaces so I don't know if this is going to cause trouble a little further down the line. Its the first time in over 40 years I've ever come across a fuse box failing, and I've had some old nails in that time. I'll be taking the faulty one apart in due course to see if its repairable.
Hopefully that is the end of this particular fault and I have many miles ahead before the next problem.