Depending on the housebuilder, they have probably finished their year-end or half-year figures and will be in no rush to get new sales/reservations.
Add in the fact that there are more buyers than new homes and the Help to Buy subsidy and I don't think you would get more than £5,000 in negotiables.
If it is a stock home (ie 100% finished) and so are the others they may be keen to get the sales set up to a new site.
Be aware, it will be much more difficult to get the snags and defects sorted once the construction staff have left the site. You will be in the hands of customer care at the regional offices who in the main are pretty useless.
It will to an extent, depend on who the housebuilder is!