Thank you for link Mamy, yes I did wonder that pixie, but my sons car is diesel, and pays less than OH and myself who are petrol and OH only has a little 1100cc engine
it's about CO2 emmissions, there are also differences based on age, 2 key dates are 2001 and 2006. Here's a useful site and app that can tell you all sorts of things from a registration.
https://know-yourcar.co.uk/
here are the actual tables:
https://www.gov.uk/vehicle-tax-rate-tables