Pump runs continuously?

I have a Potterton Suprima boiler with a Megaflow unvented mains pressure water heater. The pump has been running continuously except for a couple of seconds every minute or two. At the beginning and end of the two seconds the boiler clicks. Usually the boiler's LED flashes slowly green, during the two seconds it's flickering red. Hot water and heating are both off at the controller. The boiler stat is max, the cylinder stat can be anywhere, it appears to make no difference. Judging by the clicks when I turn the cylinder stat, the water temp is 70 degrees.

Disconnecting the cylinder stat makes no difference.

I've just disconnected the pump, the boiler still clicks for the two seconds, and fires up occasionally.

I'd like to get the water temperature down and stop the pump running needlessly. The boiler is 6 years old, with no evidence of any previous repairs.

Help please.
Thanks