Something to bear in mind re radio calls. The crew were under NO obligation to make radio calls and would have not expected ATC instructions of any sort. In the military the general principle in any event on such flights is to fly round everything and speak to who ever can give the best service, probably expecting just a flight information service at best.
They were flying at low level on an operational flight (it is politically incorrect to call the NI situation at that time a civil war) and had no option to climb into the prevailing icing conditions because of the limitations of this unserviceable airframe / uncleared type which was grounded the previous day by the MOD test pilots responsible for giving the aircraft a Release to Service. The Boscombe Down test pilots refused to fly the Mk 2 airframe in their charge 10 minutes up the road to RAF Odiham for servicing. Draw your own conclusions....