Not going anywhere, just here for a few days doing odd jobs.

After quite a lot of fuss and upheaval involving the plumbling under the bed, I managed to drain the calorifier.
Then I cut back the insulation and found where the water was seeping out through the seam between the top dome and the cylinder body.
Half an hour upside down in a dark cupboard full of fumes from the smouldering insulation with a blowlamp and some solder and the leak is no more !