[Martin]

I'm trying to improve a bed that consists mainly of nasty yellow clay. After the recent rains it is really heavy and wet. Am putting as much of my compost in it as I can spare, but will also add sharp sand (or grit sand) and gypsum, as has been advised in other threads.
Whats the difference between sharp sand that I would get from a builder's merchant and horticultural grade sharp sand?

[muntjac]

the cost , builders  sand £16 a ton . hgs a frikin lot more  

[John]

Coarse builders sand is fine, I think. On clay soils horse manure is supposed to be better than cow but organic matter is the critical thing to add

If you can get fine grit that helps break up heavy soils too. I've seen a product called claybreaker but it was very expensive so I can't say if it works.

Finally, check if the soil needs lime. Lime helps the fine particles in clay flocculate (no that's not a naughty word)  join together so lightening it a bit.

Clay is composed of really fine particles where sand's particles are larger.