In my experience there is never any difficulty in obtaining the information from DNOs for this sort of thing, as has be already stated each DNO makes their generic information freely available.
I attach ENWL's versions of a couple of documents that would be appropriate in an installation of this nature as examples. Noting that all DNO's have their own version, which are usually virtually identical across all DNOs.
The only caveat is that DNOs will follow the contract path rigidly!
The connection agreement will be between the DNO and the customer - information will be freely available across this route, and if the customer tells the DNO, "Heyyall will be acting as our technical representative in this matter " then exact information would be directed through you. This would include all specific drawings and the like for this particular installation.
They will not divulge this information to a random third party, even if you are intimately involved with the project, from the DNO's perspective, you are a random third party.