Edward W. Thompson wrote:
Have just found a drip from the bottom of my hot water cylinder so I
guess its time to replace it! I need some advice.

The cylinder is probably 15 years old and is lagged with expanded
polystyrene, I guess. It is an indirect cylinder with an immersion
heater. Looking at the catalogues it appears an identical cylinder is
available with the correct configuration of inlets and outlets. So
far straight forward to replace ;-).
snip
All guidance will be much appreciated.

I'd certainly first try to see where it's leaking from - and if it's a
place where pipes go through, to give the relevant nut another 1/6 turn
or so.
If that diddn't cure it, back the nut off, put in some PTFE, clean the
surface a bit, and tighten it back up.
They rarely - though not never - leak through the wall of the cylinder.