OH already ordered the topsoil, before I could work it out. I meant to say the compost is from the garden centre the topsoil is from a builders merchant. It was just that I wasn't sure if the topsoil was cheaper than compost, we have more beds to do yet.
So thanks for all your great ideas.
DavidT says its 40 bags and I will take his word for it as he seems to know his stuff. Therefore the compost would cost approx £120 for the same if bought in bags.
Therefore have decided to mix topsoil with some new compost I already have and some old compost. Which should help with any slumping. Round here £58 seems to be about the cheapest for 1 ton of topsoil, its actually on special offer at 'Covers' don't know the usual price.
Just noticed The topsoil is called Earth Cycle Topsoil Supreme, and that company is in Oving, which is just a 10min drive down the road from us.