Pulsacoil 2000 - not hot water PCB red light flashing 3-4 Hertz

Pulsacoil 2000 - no hot water. (I recently performed the update kit to replace o/heat stat and fitted two new stats - worked fine for 2 weeks) Now...
Sypmtoms: (over the phone remotely from user!)
Redlight on the PCB flashing rapidly (3-4 times a second)
Pump feels very hot but does not appear to be rotating.
Any clues or measurements to help the diagnosis? Customer 'fiddled' with the board-mounted fuse and reckoned the pump started. Maybe we have a defective fuse/holder. Does the fuse feed the pump?

Follow-up to the above post - after a site visit.
Red light on the PCB is still flashing - is that normal?
Checked the pump: I found it hardly rotated at all on speed settings 1 and 2, and even on speed 3 the pump seemed very weak - easily stopped with a light touch from a screwdriver on the shaft. Should the pump be stronger than this?
Fuse ok now. Hot water now present, but should I be concerned about the red flashing light and the (apparent) weakness of the pump (Grundfoss UPS 15-50X18)?

Thanks if you can comment.

Viking, Gloucester.

Check the supply to the pump, at 60v it should rotate slowly, if it does not the pump is sticking. Open a tap and inspect the pump voltage, this should increase, then check the pump is rotating quicker. If the pump does not pick up speed it most likely is sticking. If the voltage does not change, it could be the PHE sensor or DHW sensor, or worst case the PCB.