Richard many thanks for your advice, I have contacted the club and here is there reply to date:
"Thank you for your e-mail. YU 9681 is indeed a Morris 10cwt. mailvan, in fact it's a 1927 Morris Snubnose 70 cubic foot mailvan, one of a small batch of four (YU 9679-82) such vans. I will have to do some research on the first use of GPO owned mailvans (as opposed to contractors) in Bridgnorth and come back to you at a later date."
Many thanks to all who have helped out on this thread, and for those that got the van right ....well what can I say other than Hon membership forms to the Post Office Vehicle club are being dispatched to you as we speak...!!
AL