^The proportion of diesels on the used market will be increasing simply because it represents the aging of vehicles previously sold, and new diesel sales were rising considerably until recently. So e.g. the used diesel market today will be based on cars sold a few years ago. This is especially pertinent for e.g. 3 year old fleet vehicles that were first registered before the 2017 tax changes - diesel was very attractive in those days. Essentially the used market will implicitly lag trends in the new market rather than necessarily being a knee-jerk "quick everyone offload your dirty diesels". If you wait a couple of years, you'll potentially see a reversal of this trend, because of the drop in diesel sales in 2018 (and probably 2019, but I don't have stats for that).