600 miles done, got an earlier ferry. My field repair held up and 23.3mpg average pulling my trailer the whole way.
I fitted an aftermarket head unit about 8 weeks ago and it has a battery voltage display. Looking back after the fuse box burned out, I remember that it was showing 11.2 - 12.0 volts, but mainly 11.4 whilst driving. I checked the alternator and it was putting out 13.8v give or take. Since I've fitted the working fuse box the battery voltage display on the radio now shows 12.3 - 13+ volts all the time, even with all electrical items, main beam, heated windows the lot. So it was a precursor to the fusebox burning out. Note to self about voltage drops in the future