Over the last week I have covered almost 200 miles and I was really pleased to see that the engine comes up to temperature very quickly, albeit slightly higher than I wanted (The needle sits on the 'A' of 'Normal' on the gauge) then stays there throughout the trips. Fairly extended waits at traffic lights have no effect and it was only 2 occasions, in traffic queues, that the temperature started to rise. On both occasions my new adjustable thermostat kicked in, started the fan, and the temperature quickly dropped back to normal.
So I am happy now that the previous expansion bottle cap was either weak or had failed and the new stronger one is better. And that the other problems were all caused by air locks in the system, which have now bubbled out.
One other thought is that we have previously been over filling the system, which has caused the excess water to overflow from the expansion bottle. Only time will tell.
So for now I think we have it fixed.