it must be somethign to do with the thermostat as thats the only variable here.

it seems the rad etc are all doing their jobs correctly but the flow of water/coolant is too great for the engine.

change the thermostat again (not expensive) and perform a full engine flush with halfords 2 part rad cleaner and refill the system with 50/50 water coolant.

cant think of anything else to suggest really, would have said maybe an airlock, but you stated the inside heater blowers change temp so thats all working