OK, I can tell you our long and sorry saga. Best put the kettle on 
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Saw a new build we loved 5 months ago - there was just the shell of it at that stage but we really liked the area, the house was a great design, etc. It was priced at £310k and our own house had been valued at £190k.
In the interests of moving quickly, we went for P/X. The paperwork states very clearly that in the case of P/X we wouldn't be eligible for any other incentives whatsoever. The initial valuations (done over the phone by estate agents with their fingers in the air, no home visit) came back at £170 - £175k. We said no thanks, we'd just sell it ourselves and come back to them.
Months go by, we fail epically to sell our house (3 lots of buyers pull out). We keep in touch with the developer, who is having a similar epic failure to sell the newbuild.
About a month ago, they phoned us and asked if there was any way we might reconsider the P/X if they could do us a good deal, because it was the last house unsold, they wanted to wrap up their sales office etc. We go back, and fill in all the same forms, they send out estate agents, and come back with a valuation of £165k 
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However, they then said (proactively, without us even trying to push it) that they would reduce the selling price to £295k, pay our stamp duty (nearly £9k), pay all our solicitors fees (about £2,500) and give us £4,000 cashback towards carpets and decor.
On that basis, we've gone for it and (fingers crossed) we should complete in about 10 days time.
We've had to get a gas safety certificate which was £45 and it came up with a couple of minor points (e.g. the gutter on our conservatory is a bit closer to the flue from the boiler than it should be) but nothing that has caused a big issue. And we have to leave carpets, curtains and light fittings - but obviously they don't have to be your proper decent ones that you intend to take with you. I'm just replacing light fittings with those cheap paper lantern jobs.
So in a nutshell:
- Be prepared to be offered a price way way below what you would otherwise ask on the open market.
- Know that despite what they say about no additional incentives, if they want to sell it to you that badly, they may well pull something out of the bag.
Hope this is helpful & good luck.