I suspect the drop in temperature at 50 minutes (when the stat opens presumably) is a combination of a bolus of cold-ish coolant from the bottom hose and the fact that the radiator was cold and removes heat from the coolant until it gets up to temperature.
I can't explain the gradual drop in pressure from 30 minutes onwards, perhaps hoses expanding. Thereafter, I'm assuming that as the stat opens further that there is an effect of introducing more flowing coolant into the system. Possibly also hoses expanding, though this would have to be due to temperature, not pressure.
This was a no-load test. Other that the cold start and fast-idle when the rad fans kick in, it was sitting at 800rpm, or thereabouts. The +ve pressure in the system at rest I think was to do with the fact that I had performed a minute at 2k RPM yesterday - the hypothesis being that I had introduced additional exhaust gas into the system at that point and it had not vented. During this evenings test, it may well have been the case that I wasn't running hard enough to open up whatever cracks or fissures there may be in the head/gasket.
Next two runs will be:
- let fast start bring up to 50-odd degrees, then raise revs to 1k5 or 2k, either continuously or periodically, and observe.
- a road test along the dual carriageway.
I should perform a sniff test following those two tests to confirm my suspicions, though it remains to be seen how conclusive those results are.
Edit: that's also the last time I record the results on notepaper and manually enter them into Excel!
imo.