Mordo,
I think you have just cause to be suspicious. It seems to me that the
thermostat should maintain a stable operating temperature,regardless of
outside conditions(within reason of course). You did not say how cold it was
were you are at. The inside heaters are before the thermostat so running
them might have some affect. I would leave them off and the bus idling while
you wait for testing.

  Though risky, you could cover part of the radiator with some cardboard as
a diagnostic measure. If this brings up the temp needle I would say the
thermostat is sticking open.

  I highly recommend a genuine VW product here. I have had bad luck with
aftermarket versions.

Ken Lewis
http://neksiwel.20m.com/