OK, I've stripped down the aqua sensor and all seems good in there, impeller rotates very freely and no muck to speak of, the only problem was that a wire came away from the hall effect sensor connector, which I've attempted to solder back on 
Also stripped down the diverter valve and everything seems to move freely apart from the actuator, which feels a bit lumpy when rotated (though not sure if it's supposed to be a stepper motor) but not sure how to test.
Anyway all put back together and tap LCD still not coming on when DHW is demanded and not firing up for such either. So I'm thinking it's looking worryingly like the PCB
Also stripped down the diverter valve and everything seems to move freely apart from the actuator, which feels a bit lumpy when rotated (though not sure if it's supposed to be a stepper motor) but not sure how to test. Anyway all put back together and tap LCD still not coming on when DHW is demanded and not firing up for such either. So I'm thinking it's looking worryingly like the PCB
