So my wife and I want to book your services and we are keen to get you in before we complete at the end of this month. I have raised it with the Site Manager who is less than impressed and won't allow anyone in. He says it is "not your property yet" and that they won't even allow the carpet man in to measure. I wondered if anyone has any advise at all as we are planning to move everything in on completion and have a couple of kids so I assume it will be a bit manic / too late once we complete?? 

HI Frank - It is a common thing for us to be denied access don't worry. Just for your reassurance it won't be too late inspecting your house after completion. In fact we do this most of the time. 
But there are a couple of tactics you can approach in the meantime. Firstly, advise the site manager that you intend on using us regardless; as it is initially to try and put you off using us altogether. It is much easier for them to address 1 comprehensive list by us rather than addressing ambiguous lists they often get, and to do it with an empty house.
This is why it is not any company policy from any builder, because frankly we have been in prior to completion to every builder out there. 
Your second option is more formal and that is to get the solicitor to incorporate it as part of the sale. In other words, you wont complete unless it is agreed for us to go in a week before. 
I hope this helps and please feel free to follow up with any other questions