WEll, I've found out that the part is just called a flange, and that it 
used to be plastic but has been replaced with metal by VW.  Also, there 
used to be an o-ring with the plastic part, but now there's a gasket.

So, here's my next question - how much coolant is lost when changing this 
flange at the cylinder head?  I've called a couple of mechanics and one 
says you probably have to bleed the system and the other says the job'll 
take a few minutes.

I've heard this sort of thing before, so who knows the truth?

Thanks,
david