Neil,
You should perhaps contact the Heritage Motor Trust Archive,Gaydon,Warwickshire and enquire about their archive holdings.
I believe that the Mulliners records survive as part of the Standard Triumph records,held at the University of Warwick Library.
Be careful though,there were several similarly named coach builders,originating from the same family. The most likely maker of the WW2 Utility body,I would think was probably Mulliners(Birmingham) Ltd who had premises in Bordesley Green,Birmingham. They produced coach built car bodies in the 30s and during the war made aircraft parts and Utility Bus bodies,continuing bus bodies and body building after the war. (They would seem more likely than the London based H.J. Mulliner company).
The Birmingham Mulliners were acquired by Standard-Triumph in 1958 which was in turn absorbed by Leyland,the factory closing,I believe in the early sixties.
In recent years I did business over a period with a Bangladeshi bed and mattress makers based in part of the old Mulliners works in Bordesley Green Road. I enquired during a number of visits if anything remained of the old coach building business but sad to say was never once shown a dusty cupboard containing old drawings or blueprints.
As far as I know and I'm sure you are aware Stewart and Arden were car retailers but of course during the war their workshops went over to military vehicle maintenance and overhaul.
Good luck with the project,
David