Posted: 14/09/2015 at 10:17
How big an area is it? Can you fork it over? If so, try and source a large pile of well rotted compost - local council recycling? - or well rotted horse manure and lay it on thickly as soon as possible. Fork it in lightly then leave it over winter for worms to work into the soil.
Autumn is the best time for planting shrubs while they are dormant above ground and can then concentrate their energy on putting out new roots over the winter. You can plant through the mulched layer it adding extra soil conditioner to planting holes and some coarse grit to improve drainage and a handful of bonemeal or microrhizal fungi to help with root development. Both available form good garden centres.
Water in and mulch with more soil conditioner. Wait till spring to plant perennials but you can get some daffodils in now too. Not tulips. They don't like heavy clay.