It sounds like the CPU may be looking for output from a sensor.

Yeah - having spent this long on it I bet its something trivial!

This is a switchmode PSU out of a cheap DVD player:http://www.users.on.net/~fzabkar/DVD.../G1928_PSU.gif
Notice the 22uF, 400V capacitor (E1) at the bridge rectifier (D1-D4).

Thats a fair point - still not too sure why it'd need to be rated at
400v - hmm unless this is something to do with earthing.
But yes I can see it and it all makes sense.

A transformer needs at least 3 pins (auto-transformer), but usually
has 4 or more -- 2 pins for the primary winding, and 2 for the
secondary. Your component has 2 pins, so it must be an inductor of
some kind.

My original write-up was poorly written. It has two contacts on the
unsooty side and 3 on the sooty side (the side with an unused centre
tap) Sorry about the mix-up. I think it is a pulse transformer for
driving the motor.

I suspect that the triac drives a motor, pump, or solenoid. If the
track is open, then that could very well be your problem. However, it
may be that a shorted motor, pump, or solenoid is the root cause.

The motor is working fine for the wash cycle so I don't think its
that.
The PCB track is fine - tested with a multimeter just a bit dirty.

Is this capcacitor likely to be a non-polarised version or just a
plain electrolytic?

It's most definitely a polarised electrolytic. With respect, if you
have to ask this question, then you will be facing an uphill battle.

I am certainly no electronics expert you can be sure of that.
But I'm not scared of a soldering iron - well, not now I have a decent
stand for mine anyway :-)

If you can't find an online file hosting service, feel free to email
it to me and I'll upload it to my web space. Try to photograph the PCB
from both sides.

I'm not trying to aviod the subject but I cleaned the board whilst
checking for shorts and
the like so the picture would not be that interesting.
Last time I took a macro picture it was all blurry - so something else
I need to brush up on!

Now, does anyone know a resonably priced source of 400V 22uF
electrolysic capacitors? :-)

You appear to be posting from the UK (which explains the 400V cap), so
try Maplin.

Yup, Maplin was my first port of call but I couldn't find one in their
e-catalogue.
RS will sell me 5 for £5 plus postage.
But now I come to think of it, I have an old DVD player in the
garage....

I'm now thinking that I just jumped to the conclusion that it was the
electronic burning
smell whereas the problem is probably due to the filter being full of
nails.
I'll take a look tonight!

Thanks for the followup.