Had me baffled, it must have been a result of the leaking matrix there is no other explanation really, or from when the car overheated the first time, as for the cure the only thing I can come up with is that the airlock (if thats what it was) was remedied when I drained the engine down fully, both thermostats worked just one was better than the other so I fitted that one, the car was run up to temp I felt for the system pressure and the thermostat opening which it did then waited for the fans to kick in which they did then waited for the fans to stop, did this over about 10 minutes just to be sure then we took it out for a road test and all seems to be fine (touch wood) the system temp is stable there are no leaks, the system holds its pressure, the pump is flowing as it should and the fans are cutting in and out as expected, a very odd but interesting day
Yes the matrix is bypassed, it was the first thing I did this morning.