Hi y'all
Relief & good news to report

I've put this as a new thread as my original (Instrument circuit fuse keeps blowing
http://www.omegaowners.com/forum/YaBB.pl?num=1194358402 ) seems to have gone cold.
After removing the alternator, getting it checked - confirmed ok, and refitting, it was back to square one.
Decided to make close inspection of main fuse panel (behind drop-down flap under steering column).
Removed black plastic panel holding all the small 2-pin fuses, carefully turned it round as far as poss. & checked for loose wires/connections. ALL OK.
Turned back round to refit and noticed three empty fuse positions with terminal connections & marked yellow (20A) (All other empty positions had no terminals).
Secured fuse panel in place, fitted new 20A fuse in each slot and then new 15A (blue) fuse in slot 15, the one which has been the cause of all the trouble
Turned ignition with some trepidation - dash lights came on as norm. & fuel gauge moved from empty

Started engine ok, all dash warning lights went off, including battery warning light, and ALL electrics worked

Left engine running for a while and engine cooling fan kicked in - Vwalla!
Those 3 20A fuses must have been absent for the last 4 years yet this is the first blown fuse I've ever had with this car. Still don't know why it happened after all this time and the problem was resolved more by chance than anything else. Still, it
is sorted and I'm a happy Grimble again.
Thanks to everyone who contributed. No doubt I'll be back with another question before too long.

Cheers & best wishes