Hi Paul,

Trade-in values are notoriously difficult to predict and there's no simple formula, unfortunately. It will depend as much on condition, service history and how badly a dealer wants/doesn't want the deal and/or your car as a trade in. Saying all that, the car's age is likely to be the starting point for a dealer's consideration, with low-mileage examples in good condition for that year attracting higher offers. 

I would certainly urge buyers to consider a hybrid or electric car as their next purchase if they suit their driving profile, as purely diesel and purely petrol cars are very likely to fall out of favour in the coming years. 

With your low annual mileage, there's no need to have a diesel at all.