I took the tank to a proper, knowledgeable, helpful, local engineer (I think the most modern machine in his workshop is of about mid-60's vintage).
He rigged up an air pressure test ...... and found no leak.
He blocked the outlet spigot, filled the tank, and left it overnight ...... no leak!!!
He shook it up a bit ....... and hey!!!! ..... a few drips started to run from the welded seam along the forward lower join. So presumably, it was bit of a random leak that could sometimes be blocked by bits of tank sediment.
He set about the seam with a buffer and found several pin-sized holes, which he rewelded.
He then re-pressurised it, and refilled it....... no more drips.
He charged me for an hour and a half of his time.
Am I allowed to give a hearty recommendation for Andrew Snowdon, A J Services, in Springfield Industrial Estate, Burnham on Crouch? A truly decent, honest, capable, human being.
I've now refitted, replumbed and refilled: no leak (so far)!!