From MoneySavingExpert
"Law firms argue that affected cars and vans produce more harmful nitrogen oxide (NOx) pollution than advertised – and were therefore mis-sold to consumers.
In other words, you could be due money back because:
You'd never have bought the vehicle had you known about the alleged emissions flaws.
You paid more for it than you otherwise would have. For example, because you paid a premium for what you thought was a more environmentally-friendly car, whether new or secondhand.
If the car or van had to be fixed to comply with emissions standards, the fix itself may have led to worse fuel efficiency or worse performance – potentially lowering its value or creating added costs, for which you may be able to claim damages.
Of course, the manufacturers disagree – for example, VW argues consumers haven't suffered any financial loss. It'll be for the courts to decide who's right."