Thanks all, much appreciate the suggestions.
I hadn't considered the (now rather obvious!) downward bath movement
issue, which I now realise must be the cause of the 'lifting' I
mentioned.
In my impatience yesterday I bought another sealant, Homebase's
'Superior General Purpose Sealant. I used the caulking gun with an
improvised 5" extension to paint this transparent stuff liberally -
but over the existing sealant! Also around the tap 'washer'.
I should have added that I'm very apprehensive about attempting
removal of the plastic bath panel. I had to buy a fairly expensive
replacement from a 'legacy' supplier last year as the original (30
years old) was badly cracked and no longer available from any normal
supplier. I had a devil of a job getting it on and I'm worried that
removal carries a high risk of cracking it. I *think* I sort of wedged
it on, flexing with upward force from the bottom while sliding it very
slowly inward along the bathroom carpet.
Because of the difficulty of easy inspection under the bath, after
repairing the kitchen ceiling a couple of weeks ago, I invested
several days making a 'damp detector' to warn me of any re-occurrence.
(The only aspect of DIY that I'm confident about stems from my hobby
of practical electronics/electrics.) The sensor is made of bare wires
inside a sheet of blotting paper, inserted through a temporary hole in
the ceiling plasterboard and spread out roughly over the area where
the drips from the tap area occurred. Its electrical resistance drops
when damp and provides the input to my circuit, triggering a loud
alarm. That went off around 06:00 a few days ago while I was
showering. And prompted my original post.
As the sensor is in the corner directly above the gas boiler, it dries
fairly quickly, so the device is now monitoring again. My first shower
since my crude sealing was completed this morning without drama. But I
realise I'm on borrowed time!