Cracked it, in the most roundabout fashion possible.
Browsing ebay for interesting old kit, did a search for Redifon, which turned up an auction for a set of playing cards, the picture shows the back of them; "Rediffusion Redifon Rediweld Reditune".
I hadn't realised there was a connection. Given such a group, if Rediffusion wanted to market a transceiver, who would be the logical supplier? Tried a search for Redifon RT106 and up came the service manual for "Redifon Safari RT106"!
Reading it it is clear this is the unit my PA stage came from.