So, taking Basil's engine out for the clutch, amongst other things, I've removed and serviced the Heater Tap.
With reference to the composite image below....
Ever since I brought him, there's been a bit of damp seepage around the join to the head, and around the top of the tap, as illustrated by the 2 red arrows on #1. Well I soon found the seepage under the base plate was no washer under it, just some Red Hylomar type sealant.
#2 On stripping it down, the whole tap seemed to have been glued together with the red stuff and the sealing rubber on the shaft was beyond redemption.
In the plumbing world, that's the one seal I've never found available for replacement on leaking stopcocks etc.
#3 Measuring it up, I found the shaft to be 7mm and the housing 11mm, so I brought from the usual auction website, a pack of 5 7x2 O rings, that's 7mm internal diameter and 2mm rubber diameter. So 7mm + twice 2mm = nominal 11mm external diameter.
After cleaning off all the grime and rubber residue, two O rings was a good fit for the Shaft seal.
A little PTFE thread tape replaced the copious amount of red paste on reassembly.
It's now just awaiting refitting.
If it leaks around the top, I can just just tighten the top nut a little more, or add another O ring.
One rescued Heater tap

Just in case anyone in the future is having problems with a leaking early style heater tap.
