heavy clay frost pocket

My new garden is a bit of a blank canvas and now I know why. Half of it has very poor drainage judging by the moss and it produces lots of fungi in the autumn. I think there may be some pretty solid red sandstone just beneath the surface because it's not long before a fork hits something solid. There are a few things in there but round the edges, the buddleia (sp?) is weedy and rather sad but the other shrubs are ok. The rest of the garden does seem to have more soil but this is heavy red clay - so obviously I'll have to open it out with organic matter but while I'm producing that (I can't afford to buy too much in) any suggestions as to what does well in such conditions? I've ordered bog plants like ragged robin, meadowsweet and so on. Dog roses, rosa rugosa are also in plus native shrub plants. I should add that to add to the general joys, we're in a serious frost pocket. I can't check surrounding gardens as nobody seems to trouble much, opting for large areas of grass, apple trees sometimes but not much else. There are two 'veg' men and I shall take advice from them - but they've been at it for decades and have had a good long time to improve their soils. Raised beds are one option, I know, and I've got plans in hand for them to go in asap. I want plants good for birds and insects, don't mind if their native wildflowers or cottage garden types as long as they can stand heavy clay and the waterlogging and baking clays suffers from and frost, lots of frost. Any suggestionjs greatly appreciated. Oh, the thin soiled area is shady and the rest gets good sun in summer I think.