Dodge Spacevan Royal Mail
I don't remember so many of these in Royal Mail livery, I recall the Sherpa being the van used by the Post Office. Photo looks to have been taken in the same yard as the Telecom van.
I also recall British Gas used them, but the main memory I have is them being the favoured choice of the chap who ran the foam stall on the market in Sudbury, in the late 1980s.
Scanned from a late 1970s brochure in my collection.