It's a long job with clay,several years,but it will never convert to a good friable loam. You'll never buy enough potting soil to do it unless you buy lorry-loads. You can sometimes get top-soil from new construction sites when they clear and level the ground,but of course close to home it could be the same as you've got. Lime is the big friend you have,plus as much leaf mould as you can get or make,and all the grit,sand,and compost you can muster. The big problem with clay is the tiny particle size, trapping water and preventing aeration... Grit and sand help with aeration,despite some people's objections to it..it all helps,and you won't make bricks or solid cakes out of it with enough organic stuff going in as well. Increasing the average particle size is a major step in dealing with clay,so ignore the objections..it really works... Growing potatoes helps to break it too. The easy thing is to grow roses. They love it. The National Rose Society showground is just down the road from us in St.Albans,and we're in the same clay-ridden valley in the Chilterns just north of there. Fifteen feet of it before we get to the chalk underneath. Fortunately,the Chilterns are all made of chalk,which gives us some free help,nearly as good as lime,but we have to get it from miles away where it rises above the clay. Sackfuls of it have gone in to the veg area,and thirty or forty bags of leaves dug in every autumn with more spread as leaf mould in the spring and carried down by worms. My father won prizes every year in London at the National Rose Show,from 300 bushes grown on untreated clay,with leaf mould and bone meal round the base all around....the rest of the garden was enough to deal with for veg and flowers,heavy going,year after year, so the roses have done us a good service. It isn't all bad news with clay though...have a look at the clay water meadows and drier land a bit elevated....there are loads of types of grasses and wild flowers growing on them. All with no human help. Not a fork in sight.