Hi Yorkie,
We had the same problem, where we used to live.
Added Top Soil, Peat (before the environmental issue) and good compost (not the cheap variety), later added our own home grown compost.
It's hard work, really hard - but worth it in the end. Don't expect to do it all in one day - you'll break your back!!
Good factor is, that although your garden can resemble a quagmire, clay soil has wonderful nutrients in it, and roses (so the experts say) love it!!
So good, that if you dig a hole for a pond, you don't require any liner!!!
good luck
n00dles