LTNman: OK, it could be NP229, which is still a Mark IV. Was this Perspex dome a regular feature? I just Googled "Percival Proctor", Images, and see the dome with a DF aerial inside on a few of the photos (and a model), but most do not have it.
I'll also take this opportunity to say what a fantastic thread this is: it's one of my regulars.
Laurence