By Nikpannu
United Kingdom 
First of all thanks for your responses, seriously amazing help. One more if you don't mind. Aswell as topsoil I've been offered a load of rotted horse manure for free by a local farmer friend. If I rotavate this in with the topsoil will this encourage grass seed growth.
Thanks again Nik
14 Apr, 2016
If the manure has been composted (preferably with straw) for a year at least, preferably two years, yes, it will encourage a good lawn. But I will take this opportunity to clarify one thing, one of the basic principles of growing plants, including lawns - it won't directly help your seed to grow - what it will do is increase the fertility of the soil, which means anything growing in it will be able to find the nutrients it needs much more easily over a longer time period, so it helps indirectly. Chemical feeds such as Growmore or lawn food break down within 1-6 weeks, depending on formulation, and supply nutrients for that time period to something that's growing - but they do nothing for the soil, so long term, its best to use organic materials like manures wherever possible and appropriate. Its about feeding the soil rather than feeding plants directly...
But what I've just said is not why I didn't recommend bonemeal or other fertilizers - its because chemical feeds are usually too strong for grass seedlings if you're going to sow immediately, so in this case, your only choice is to use organic materials such as composted manure mixed in - but don't overdo the manure. It just happens to be the best choice as well, because this is the only opportunity you'll have to get some organic materials into the soil - once the lawn grows, it'll be primarily chemical feeds all the way.
And, by the way, buy the best topsoil you can - a lot of the cheaper stuff is motorway spoil with the larger rocks screened out, which means its full of weed seeds and pebbles... and be aware that the manure you can have from the farmer friend is also likely to be full of weed seeds. to be frank, in such circumstances, I'd be inclined to work in the topsoil and the manure, and then wait 4 weeks before sowing grass seed, to see what germinates in the mix you've spread and deal with it before sowing.
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14 Apr, 2016