Got to help a relly out -

Couple of questions:

Current one is sealed in with some sort of gunk on the bottom entry
hole. After I've scraped that off, what's a recommended sealant to use?

Fernox LS-X or something with a bit more "stick" like Geocell TheWorks
(MS polymer I think, stickier than silicone) - or something else.

The other thing - do all bottom entry valves have the same threaded
section length? The pipe runs straight in with a tap connector on the
isolator valve so there is no "give" for slight differences.

Sorry to sound thick - only used side entry valves before.

I'm thinking of a

http://www.screwfix.com/p/fluidmaster/47792

as I have one at home and love it. Brass threads and a good clean
shutoff. Not sure yet if there's room, as the fill entry is on the same
side a sthe flush handle so it's a bit crowded in there!