Yorks.ppl,
I, too, used to fly those Warriors quite frequently, and suspect that Northern Highflyer's theory is the correct one.
Ask at the Sherburn fllightdesk for a copy of the relevant pilots guide manual, one of the instructors readily gave me a copy when I started hiring those a/c. (Or there may be one available from the web.)
The receiver is a KNS80 as far as I recall.