I looked up a diesel specialist in Norwich and found EDS in Vulcan Road South. I cycled there with the filter bowl to see if they could supply a drain plug. They couldn't, but the helpful man disappeared into a back room to emerge a few minutes later with the bowl fitted with a steel bolt, nut and copper washer. He'd also retapped the thread and levelled the face where the washer meets it. Total cost? "To a fellow cyclist, no charge". Excellent! If only I'd remembered to get the other, spin-on, filter while I was there. (Although they seemed to be more an engineering place than a parts place.)
Although the bowl is now useable by tightening the nut against the washer, I think I'll cut the bolt down to size so as not to need the nut.
Tony Brooks's technical training notes, I've worked out that the unit I've been working on is a combined filter/agglomerator. My engine appears to have no sedimentor, which would have been put between the tank and the filter/agglomerator.
Now all I have to do (!) is return to the boat and put it all back together again. And change the other (spin-on) filter. I think I ought to do this before our next scheduled cruise as that's not for another six weeks or so. A full tank of diesel is relying on one stopcock to keep it from flooding the bilge.
Oh yes: while I was in the engine hole I did another thing new to me - I tightened the stern gland a little to reduce the dripping. The gland was dripping at the rate of just over one drip per second even after squeezing grease in, so I gave each bolt half a turn. This seemed to do the trick.