The plastic tank (put in by someone ages ago) will now be removed and
sit in the loft, on a wooden board 40mm thick. I don't think it needs to
sit on special insulation.

There is about a six foot length of plastic padding stuff that is the
insulation. Also a two foot long piece as well.

I think it's up to insulation requirements, if the six foot length wraps
round the side of the tank and the two foot piece sits on top.

http://www.polytank.co.uk/assets/pdf...nk_fitting.pdf

Of course I must also insulate all the pipework.

Please, anyone, let me know if I'm not insulating correctly, but
according to the link, I think I've got all the the insulation that
originally came with the tank.