Hello Graham,
Not sure how much of this is going to be helpful since you seem to have covered most of what would be seemed logical to cover, so it may be time to further question the way in which you have done things. Apologies if this appears pedantic but this is what I would do I terms if rechecks:-
With coolant at cold, turn the heater control to max. hot and drain the system (collecting the coolant if you can). Remove the thermostat and place in very hot water on the stove, and continue to heat the water until the thermostat is open. Carefully remove the thermostat from the water with a long nose pliers and insert a length of cotton between the body and the open valve (be quick!) The valve will now close and jam the cotton, and you can suspend the thermostat by the cotton thread. Armed with an accurate thermometer, gradually reheat the water - with stirring- and note the temperature at which the thermostat falls from its suspended cotton. This should be 88c or very near to this. Simply placing it in boiling water and making sure it opens and then adding cold water to make sure it closes is not accurate enough, You need to actually measure the temperature at which it opens, and assure youself it is 88.
When the system is refilled, as you know, you will need to squeeze the hoses to eliminate as much air as possible and then bleed the remaining air via the bleed screws. I don't replace the reservoir cap at this stage, but start the engine and allow the coolant to rise until the thermostat opens and the top hose gets warm, at which stage I replace the cap and allow the engine to run until the fan cuts in - keeping an eye on the temperature gauge. Obviously making sure fingers and fan are kept well apart, I continue to squeeze the hoses, particularly the heater hoses which should now both be at the same hot temperature. Leave the car to thoroughly cool without slackening the cap. Top up when cold and road test the car with the heater still at hot.
If the thermostat passes the correct opening temperature test and the coolant temperature still fluctuates then it's head scratching time again!
Water pump faulty? Not had one do that yet and obviously agree with Trevor.
Probably a lot of rambling and maybe not a lot of help, but...
Kind regards,
Gareth.