Hmm, well sorry to have to tell you this, Pdb, but the stuff to have bought would have been horticultural grit, not sand, but as you've got it, just chuck it about and work it in. However, the addition of well composted materials will make a massive difference - usual procedure is to add as much humus rich material as possible (soil conditioning compost from the garden centre, well composted horse manure, chicken manure, leaf mould, anything like that) along with grit.

Thanks Bamboo, Just looked on the bag and it is a J A Bower's and it says sharp sand, will this still be the grit?
The problem is I have a lot of large borders and I have used all my compost on 2 of the borders.

No, it won't be grit, grit's grit, but sharp sand at least is a bit grittier, if that makes sense, lol! Just add composty type stuff to it when you get hold of some later on. You can buy it at the garden centre, and I think most local councils sell it now too. I don't mean potting compost, by the way, the stuff I mean is sold at the garden centre as 'soil conditioning' rather than potting compost.

I've got clay soil in a relatively new build home, so had the same issue.

As a novice gardener I added various types of sand when turning over soil for some new beds. After adding it I watered and waited a day, then turned it over again. Seems to have made a significant difference.

As for quantity, just keep adding till you see it change.

I guess it's also somewhat dependent on what you want to grow there - think about how much drainage do you need etc.

As per other posts here grit will help and compost will not only help break it up but will also add much needed nutrients.