Fieldsport, Traverse City, Michigan, had a 28 bore Purdey with 25" barrels for sale a couple of years ago. Bryan had it for at least 2 years in the rack. It was a beautiful gun and it was priced right but was a hard sell because of the barrel length. In addition, it was built in the late 70's when the quality of a Purdey was probably at its lowest historical ebb.
I think Purdey only has one qualifying issue when they build a gun and that is that it must be built with their "self opening" system for a double gun. They, also, will not build a 2 inch, 12, these days.
