@Rae947
Presumably an architect designed the conversion kit. The plans would specify what materials were to be used, including the insulation. The finished conversion would then have been approved by your council building control officer. That would prove to the surveyor that there is insulation.
You should have these documents. If the conversion was done before your ownership, the documents should have been provided to you when you purchased. If not, your solicitor/conveyancer has been remiss.
If all else fails, the internal wall chadding, plasterboard or other lining could be removed in a few small, key places to give access to the internal construction and the insulation. The surveyor ought to have told you all this. I would try a different surveyor next time.