Hotpoint WD420 - replaced dead heater, now error F7 shows, flashing 2,3,4

[JimboMarimbo]

Washer/Dryer HP WD420 water would not heat up, so pulled heater and thermistor out - both dead
Bought genuine HP replacements and installed them today.
Now machine does not fill or go on to wash - just displays rapid on/off led flashing, together with flashing leds (from the top down) 2, 3 and 4.
Elsewhere on this forum I found this to be fault code F07 - Heater Relay.
This makes no sense - I mean, prior to replacing dead heater and thermistor, the machine would wash ok, but with cold water.
Am I reading the fault correctly? If so why have the brand new parts made the machine unusable?

 

[Echo the husky]

Check all the heater wiring and connections, also check operation of pressure switch.

 

[JimboMarimbo]

Ah, the pressure switch - that round thing in top of machine with a pipe going in to it? Someone else suggested blowing through it. I'll have a look tomorrow. As for wiring connections - they're all good, I checked them myself when installing new heater assembly