Well, getting back to the RF-Tech....I've fixed it.
After tracing out the circuit, no easy task with all the SMD
components, it became obvious what the problem was.
For those interested, the RF section works around a one transistor
oscillator with a SAW device setting the desired frequency. The data
stream is fed into another transistor that is connected between the RF
transistor's emitter and ground. This turns on when the data stream is
present which then switches the RF stage on.
In my unit, when it got cold, the switching transistor stopped working
due to a bad joint on one of it's leads. This was proved by shorting
out the collector to emitter which then produced a good carrier at
433.86MHz.
If anyone wants anymore details drop me a line.

Bob