Torrent Eco OV hot water tepid

I installed a Torrent Eco OV just over 2 years ago and it has performed very well. Now suddenly the hot water has gone tepid - a steady 39° - 40°. The store is definitely hot enough - 80°; all 3 sensors read correct resistance for temperature; the pump operates with steady green neon. I haven’t got a sufficiently sophisticated meter to be able to test the duty of the PWM output from the PCB. I have removed the plate heat exchanger thinking it might be blocked on the store side, but it appears to be fine. The DHW side is perfectly clean because a water softener is fitted.

Can anyone suggest any other diagnostic checks I can try or possible causes before I start buying expensive parts (pump or PCB).

Thanks.