i have just bought a series 2 with a similar problem! drove home and temp needle hardly got going. took thermostat out and it wasnt opening in boileing water, fitted new one and its better but still not great? heaters are like luke warm and as soon as the car moved temp really dropped down.

Never heard of the plastic bit you mention above?

have flushed the system today and will do again tomorow and refil everything. As above if pump was fooked engine would overheat?

What about the actual cooling circuit could this be wrong, anyone one have a diagram of how it shoud be?