As crazy as it sounds, I'm buying a new build home

. After the nightmare with Bett Homes (who are utter rubbish btw) on the last one and all other the problems I’ve read about on here I wanted some advice for the next one.
The completion is due for July/Aug (this year) and I want to make sure I get my moneys worth and no exchange takes place until I’m 100% satisfied that I’m getting what I paid for. What I would like to do is have a walk round before the date of exchange (something I fear the builders will be reluctant to do) but I want to try. When would be the best time to do this, how many weeks before the exchange is due to take place? I want to be deemed as being reasonable in my requests, any advice on reasonable action I can take before exchange to ensure the quality? If I get this walk round what should I be looking for? I’m thinking that if I get the walk round in enough time it would be reasonable to have them fixed before exchange.
Also, would it be a good idea to hire a snagging inspector?
Thanks in Advance
Clare