Thanks for all your advice. Just spoke to the architect and he said he would call the SE to clarify expectations of them both. It seems he is hesitant to start building reg drawings as he doesn't want to have to re-do them once the SE recommends specific/additional supports etc.
Reluctant in binning off the architect as it would cost me more money, as his quote said drawings would only be provided in PDF and CAD drawings would not be released.
Let's hope they can agree a way forward this afternoon. This is a two storey extension after all, we're not building the Shard ?
Will let you know the outcome.