ACW:
H.337 dated 18.4.1980 is entitled: Radio: "To Introduce Plessey PTR 1751 ARI 23301/26 UHF in Lieu of PTR 170 ARI 18197/1 UHF", so I don't think this covers fitting a VHF radio.
I have a complete modification list (available to download on the DHSL website), and I can't find anything about fitting a VHF to RAF Chipmunks after 1965 (but it obviously happened; look at the above photos)!
Any ideas, Coff?