Posted: 17/03/2014 at 19:29
when I moved into my London garden flat, the garden was probably in a similar state to yours. Very overgrown shrubs hogging the light, little else, soil very compacted and indeed sort of "sick" looking - yellowish. I double-dug one border, but having a bad back never really got round to the rest. I just "tickled" the top of the soil to open it up a bit, and in the first year, chucked loads of compost on it, topped with newspaper to thwart the weeds and covered with bark for appearances sake. I then just dug nice big holes for planting. Since then, I have just chucked organic matter - compost, leaf mould, manure, whatever - on the surface and let the worms do the work. Clay is actually very fertile. As long as you dig big planting holes, and add plenty of compost, most plants should thrive (unless they like sandy soil).
It can look pretty horrible when you hack back overgrown plants, but things will grow really quickly now and in a few weeks even, it will look better. Of course, having cleared the ground and increased the light, you will have to contend with weeds - they will have been waiting for such an opportunity and will germinate in spades.
Depending on how much light you have, I would recommend quick-growing clematis to clothe the borders and then think about a few choice shrubs. I can recommend Daphne - pretty modest most of the year, but gorgeous scent in winter and early spring. Doesn't grow too fast or too big either. Then maybe some euphorbia (they need a bit of sun, not necessarily all day). My euphorbia melifera (well, it's spelt something like that - honey-scented spurge) is just coming into flower now, and again, the scent is knock out, but there are other wonderful varieties with various outstanding features - there's bound to be one to suit, unless you garden in deep shade.
Philadelphus and deutzia have white flowers and lovely scent later in the summer. If you are on a budget and are patient, you can buy these very cheaply in several high street budget shops. Then think about perennials - hardy geranium will cover the ground and give you months of flower power.... but really, the list is endless. If you have a sunny border, enrich the soil a bit, buy some annuals and scatter them - you will get some quick-fix colour while everything else gets going and you think about longer term planting.