are these interchangeable with other vag groups cars - I've got one for a 2.0 tdi (part number 03G121113) and wonder if it will fit my 1.8T?

no it won't...diameter ,depth or both are different

 

part no for VRS stat is 050121113C

are you sure - the gates aftermarket replacement seems to fit many, many types of car....

do whatever you want

I bought the GATES TH00188G1 stat from Amazon which is listed as being a compatible replacement for both the above mentioned part numbers.

 

Thermostat seems fine but did seem to stick open for about 2 days about 2 months after fitting.

thats what I meant, surely if the gates fits both the OE part must fit both, no?

Try it...