Funny the Revenue details for that model show a NoX value of 29mg/km working out at €145 on top of VRT.

The only real way to tell is to check the V5 document (or get the seller to check it for you). This is the document that revenue will use. It's detailed on Section V.3.
It's a bit of a minefield.
Luckily the little 2003 325ti is only 17mg/km so only €85