Ideal Turbo Overheat thermostat won't reset?

After draining and refilling CH system the overheat light came on when boiler fired up (Ideal Turbo 2 - 24).

I have now checked and vented / refilled system and all is well with boiler except that I cannot reset the overheat thermostat. I have located the reset buttons, (awkwardly inside boiler!) but pressing button makes no difference - overheat light still comes on.

Ideas gratefully received.