I began trouble-shooting by first turning each light switch off and resetting the mcb to see if I could isolate where the fault was. When that didn't work, I physically removed each light "in case" one of the self switching lights was faulty... no joy there either. Finally, I removed the mcb and swapped it with the one from the next circuit (which works fine) on the off chance it was a faulty mcb. No - that (the swapped over one) tripped just the same once connected to the aforementioned circuit.
This can really only mean one thing - " somewhere" between the front and the back of the boat, a nail/screw has gone through a wire... this is a pain as we've already applied 2 coats of varnish (1 Saturday and 1 yesterday morning) to the entire boat. As far as I can see, the roof lining will have to come out - possibly even bulkheads for "them" to try and find the problem . I've emailed the builder last night and when it gets light out, will give them a call to ask them to send someone to take a look - they'll be narked of course... who wouldn't be but it IS new and WE'VE not yet put a screw/nail or hole in her so it's NOT our problem. I won't be arsy of course - these things happen and I'm sure it can be sorted out... I just hope (given the dark nights) sooner rather than later.
The Morrells varnish/lacquer by the way was very easy to apply and barely darkens the ash.