'Top soil' as sold in garden centres is good soil for adding to borders but is v-e-r-y expensive. 'Top soil' from building sites is usually sub-soil taken after the true top soil has been skimmed off. It is of poorer quality but can be improved with the addition of humus.

We had to buy lots,when we first moved here,after digging out all the builders rubbish they had buried underneath ! plus lots of clay..expensive ,yes..but 11 years on,just added home made compost each year,and/or spent compost.....we found Wickes's own make was good quality,,,watch out for special offers..sometimes 3 for 2 or similar..

Just a thought here could you try your local council refuge centre, do they sell cheap compost? here in derby all the garden waste is selectively recycled and and made into humus rich compost and comes in at around 1,50 a bag so maybe it could be worth looking into.

I have found a company near me that sells organic black soil bagged for £37 a ton. I'm going to get 3 tons but if I buy it loose its £28 a ton but would have the problem of where to keep it. I remember enquiring at our local council and they don't have compost to sell.

I wouldn't touch anything from a recycling centre with a barge pole. You just don't know what has gone into it.
pay that bit extra for a reputable on line supplier of top soil.
I can heartily recommend this lot

http://www.gardentopsoildirect.co.uk/

Take your pick. Best stuff i've ever bought.