Emmissions MOT failure

To a certain extent it will depend on what engine you have - fuel stored in the tank "for a few months" is more likely to affect a petrol engine than a diesel. But TBH if it's been only a few months, I doubt it's that. I guess it's fair to assume that an R reg Boxer is fuel injected.

If the emissions are as high as they suggest then I'd want to be sure that the air and fuel filters have been changed within living memory, and then it's ~probably~ going to be down to a faulty sensor somewhere if it's drastically overfuelling. Temperature, MAF, fuel pressure - it could be any of those.

The OP hasn't exactly been forthcoming with information, so maybe we'll never hear anything from them about this again. So we still don't know if it's petrol or diesel.

But as far as I can see, the base vehicle dates from around 1998, when carburettors would have been rare. As far as I can see, all Peugeot Boxers from 1993 onward were fuel-injected, whether petrol or diesel.

That itself doesn't mean that a reaction to the higher ethanol content can't have happened, though from personal experience (owning both a 1960's and a 1980's car) the likelihood of damage does seem to be a bit less than some people predicted.