Lots of cursing etc.
Rebuilt the oven using some stripped wire wrap wire in place of the fine 'not tin-able' wire, joining the crystal to the outside world. All back in the chassis.
The heater control circuit is a most unusual design in that it is not a DC sensing voltage but a 300kHz signal, derived from Q1 on A25. The wheatstone bridge type sensing circuit has resistive and capacitive elements.
I have tested all of the components around Q1 ( the oscillator) and Q2 the buffer/driver and can find no fault (pulling out both Q1 and Q2 and all of the caps as well). It is NOT oscillating. This absence of oscillation allows the heater circuit to switch on fully and stay switched on, certainly getting over 70C.
When I assembled the oven I buried a Fluke temp probe, it is on the outside surface of the heating coil, and did some measurements of the thermistor again. As the crystal should be at 65C or so, the thermistor (NTC) measures about 900 Ohms at 65C. I decided to try my hand at building a new heater controller. A LM358 op amp circuit with a voltage divider, 10turn trim pot, driving a TIP33C, has the oven heater up to 65C and cycling on/off about every 20 secs or so when up to temperature after 15mins or so. About 0.5C temp variation is seen on the outside of the heater coil; inside the oven (3mm think aluminium and then more mineral fibre insulation before the crystal) should result in a pretty stable crystal temp.
Once the crystal got up to a modest temp, I experienced the "Joy of Oscillation" (- apologies to the Solder Smoke guys) of the main reference crystal oscillator circuit.
Will build the new heater controller into the 5245L in an unobtrusive manner, and do some final testing / adjustment.