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.
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.