Posted: 12/09/2014 at 09:17
make sure you sow onto clear soil and not onto grass (I work with community groups and often see them throwing money away by putting expensive seed straight onto lawn)
also the poorer the soil the better, if you can take off any fertile top soil it will help. break the clay up and add a bit of horticultural sharp sand (or the normal stuff that you've washed works just as well) get it nice and flat and sow either now or in late March.
make sure you cover it or the birds will eat all the seed
I would also go for the 100%wildflower mix as grasses will arrive on their own