*As you are in the UK, try the CPUK forum. *Someone there may be able to help you. *There's also a gentleman there by the name of Plantsman who is selling a peat free compost mix that you might check into if you can't find sphagnum peat moss.
Check the bags of peat lable of contents to see if it is New Zealand or Canadian sphagnum peat moss. Both are good. You may be able to get long fiber sphagnum (LFS) instead. It's unshreaded shagnum moss. Sphagnum is sometimes called spagnum in the UK, I have noticed. Hope this helps.