Most of the comments are exactly correct.
Boss = missus!!
If I'd had the relevant tester(s) to hand, I would have done the tests suggested at the start. But since it's my own system, I'm a heating engineer, and there were no grounds to suspect any fault and I wasn't about to quote a fixed price to myself, I didn't test it. (Being a heating engineer, I have a Flue Gas analyzer etc. but not adequate electrical test kit to prove a system!)
Yes - I know that using RCBOs to 'test' individual circuits is not a complete check (or any check at all!) BUT with them in place the installation is 'safer' than it was before. There is also continuity on the CPC everywhere (especially on the power circuits) and I will get my electrician mate round with his test equipment when the dust has settled.
Meantime, by the simple expedient of disconnecting all the pendant fittings, the N-E fault has vanished! Now just a matter of rechecking (with a bit of daylight) exactly where the problem was, and reconnect the circuit at the CU.
Thanks for your help.
And, by the way, I don't consider that I've done anything that was even slightly risky, let alone 'dangerous' during the work described and I certainly don't consider myself to be a 't**t' even when working outside my own skill set. I would however be quite concerned if I found 'BAS' had been working on gas. With that attitude, there's every possibility of a risky assumption about required skills!!