Thanks for all the replies - I'll give a thicker version of the PTFE
tape a try when I get some.

The towel rad is new and splitting should not the case... I hope.

The tread of the valve tail I think is probably parallel rather than
tapered, however I'll take a closer look when I get it off again.

I did try leak sealing stuff but it didn't work - I have seen that
fail for others in the past. Getting the right PTFE tape is now
obviously essential to the job going right - the cheap stuff just will
not do.

Thanks for the help again.
Tom