Really good advice above. But if you want to cheat a bit, I used https://www.thompson-morgan.com/p/vitax-clay-breaker--25-kg-box/6CB23TM https://www.thompson-mo

Really good advice above. But if you want to cheat a bit, I used https://www.thompson-morgan.com/p/vitax-clay-breaker--25-kg-box/6CB23TM https://www.thompson-morgan.com/p/vitax-clay-breaker--25-kg-... on my heavy clay garden and it helped a lot, in combination with extra organic material etc.