appalling soil, want a lawn..

You can sow seed from around September ... which isn't that far away. Lot cheaper than turf. (Seed is much better sown in Autumn than Spring - much less maintenance in 1st year as a consequence of Autumn sowing)

 

When I did my lawn the site used to be part of an old garage base and the soil was terrible. Not only that but I had tons of builders rubble, plaster and massive slabs of 6' thick concrete that were covered in asbestos buried in the ground (it was deemed cheaper to do this than to pay for me to get rid of them) but the ground was thick claggy clay and drainage was an issue.

I'm not sure how big your lawn is going to be but mine measures roughly 10ft square. I only used one ton bag of top soil the whole area to cover the thick clay as lawn roots don't penetrate as deeply as other plants. So I only had a covering on the whole area of about 3inches thick and then seeded it. Haven't really had any problems since the lawn has got going. I personally think 30cm is a bit excessive especially if it is on a large area. I put a lot of effort into breaking up the soil underneath as much as possible by hand though, sifting it seems pointless. Just remove the large lumps, otherwise you will be there forever. Once the top soil is down and the lawn is growing the worms should come and slowly break down the compacted soil further, at least this is what I found.

 

I think TheBatman has hardcore at present though JJ? If so that's rather different to builders rubble mixed with subsoil. My drive is made of just hardcore ... its almost as hard as concrete and nothing will grow in it, and a thin layer of topsoil over the top won't be enough. The hardcore could be broken up, but if left as a layer like that, even mixed with some soil, I would expect it to compact again