Hi Noz,

What I forgot to update on this thread (I believe it got added to another though), was that I eventually traced the intermittent blower issue to the actual plug / loom supplying the heater.
The wiring had overheated near where it entered the plug, and it seems it was just no longer capable of carrying the necessary current most of the time. Wiggling the wires and keeping the switch on a low setting was enough to make it run for a while, but as soon as it was turned up to a higher speed it would die.

I did think the same as you (i.e PCB / solder joint issue), until I stripped the lower dash and saw the melted looking wires.
I couldn't source a replacement plug from anywhere, so had to cut one off a scrap series 2 car. When I went to solder it on, I discovered that someone had been there before me and done the same thing already!

That was 18 months / 2 years ago, and its worked Ok in that time, though recently a few times I've noticed it suddenly turn off if used on setting 3 or 4 for long, so I suspect the plug may well be due replacement again...

Cheers

Ciarán