I solved the electrical issues eventually. What I believe happened is that I fitted an alternator that was correct for the year but not correct for my car. Seems that NA alternators were self regulating but NB alternators are regulated by the ECU. Since my ECU has been replaced with a mega-squirt I need regulation somewhere and I didn’t have it on the test drive that blew the electrics. So lesson learned and we are back on the road.
In fact the five has been round Oulton Park and Castle Combe since and worked well. Both these track days were wet though, especially Castle Combe but I pressed on and had a laugh once I’d gained confidence in the grip.
Clocked up about 80 miles round Castle Combe and similar at Oulton.