He did mention the build over, and I have applied for and received confirmation of my build over agreement.Well hang on, why have Building Control said you can do what you want and not mentioned anything about Build Overs, that seems pretty lapse on their behalf, you should check by reading the guidance on the water company website before making them alert to the situation, the websites usually have everything you need to know with regards to what you need to do get an agreement. Don't forget if it arises that a build over was not agreed when you come to sell that can stir up issues too.
I just had a look at this again and you’re right with regards to the 500mm clearance needed. Not too sure why building control said this wasn’t actually needed as we were discussing it… I was mid dig so rather than referring back to my emails I took this as a bonus and decided to make use of the extra space… possibly a costly mistake, however if as Marty has said above is correct and the manhole is my responsibility, might be best I just don’t mention it for now and budget to have it moved if it comes to it.