I returned to the problem yesterday....
The radiator drain tap had continued to drip, very slowly but persistantly. I also remembered fitting a similar new drain tap to the block during the rebuild. Sure enough, this was wet with coolant. This is a nuisance
I removed both, dismantled them and could easily tell that they were leaky with a readily available vacuum test that requires no additional equipment (The parts got a good wash first anti freeze doesn't taste so good). The lapping compounds I had (for grinding valve seats) were too course for this job, and left grooves on the brass when I tried them on the old drain tap that I'd kept on hand. However, although it was slow, Brasso seems to have done the trick - adding a little, working the parts together for a few minutes, then looking to see how far well contact had developed between the mating surfaces. Eventually I managed to get a substantially better match between the parts of both taps. They were then thoroughly cleaned and reassembled with silicone plumber's grease.
I'll post back in a couple of days with progress. I had no time to even run the car and bring it up to working temperature yesterday, and it needs a thermal cycle or two to give a proper test.
Happily, the resoldering work I did to fix leaks from the radiator are holding nicely.
Ed
I want my MGB