After some distractions, I finally got a chance to look some more at
our Impreza's overheating problem. Initially, I thought the problem was
electrical as none of the fans come on but it is something different, I
think now:

The fans don't come on but can get them to come on by putting a
variable resistor on the thermosensor - so the fans not coming on is
not due to any of the relays, wires, bad fans, etc. The ECU clearly
works ok as well.

Replaced the thermosensor - still the same problem. Measuring across
the thermosensor at temperature I get 290 ohm, from the looks of it.
Using a potentiometer, the fans come on at around 250 ohms.

So I am wondering if the coolant is circulating. I don't dare run the
car too long, but the bottom hose does not seem to get very warm. Took
the thermostat out and tested it in a pan of boiling water - it works.

I've tried burping the system by rolling the front on ramps and revving
it while filling the radiator. When that didn't help, I drained the
whole system, filled the engine first (using the top hose and a long
funnel), then the radiator -- still no joy.
I was half thinking of finding a steep incline and trying to burp the
system but that just seems extreme.
Is there a better way to burp the system?

How can I tell whether the waterpump is bad? I replaced that thing last
year when I did the timing belt. Usually don't waterpumps start leaking
when they go bad?

The car runs really well antifreeze does come out the cap eventually
when it overheats.

Tia
Remco