What to use

[Essexboy]

My plot is heavy stony clay, last year I used horse manure to try to improve it, result loads of weeds. This year I propose to use sterilised compost from The Green Waste co.£1 per bag ( fill your own) . Question will this help to break up the soil ? I appreciate sterilisation will probly destroy a lot of nutrients.
Your comments please

[veggiechicken]

What will you do with the green waste?
Spread it? Dig it in?
Soil is "broken up" by worms, roots and digging.

[nickdub]

Any organic material will help to make the soil structure better for growing things in it.

The good news is clay is rich in nutrients so when you can grow stuff it should grow well, but timing is important to sowing/planting etc as the soil can harden very quickly as it dries in the Spring/Summer

You might want to consider a green manure crop to be sowed in the Autumn, such as field beans - this will help the structure of the soil and also make the nutrients in it more readily available to other crops.

[ameno]

Sterlisation doesn't destroy nutrients. It doesn't get nearly hot enough for that.

Any organic matter will improve the soil structure. It needs adding yearly, though, as it quickly disappears and the soil will revert to its original state.
Adding sand will give you a more permanent improvement. Lots and lots of sand.