I covered this earlier I think possibly in another thread but the issue here is that 5-10 years ago diesel was massively more popular due to the CO2 clampdown, especially so in bigger cars like estates etc. So there is a much much smaller range of petrol cars available and hence prices are relatively more expensive. In fact if you take a given make, model and trim level of car, and compare petrol and diesel engines with the same power output, you'll typically find the diesel is more expensive list price by perhaps £2k but when buying used, the petrol will often be more expensive due to being rarer and the general distaste for diesel these days.
So generally speaking, there are significant benefits (in relative terms) from buying diesel in cars of that age range as they have depreciated more and have extremely low tax. If looking at cars from the past couple of years, these benefits fade away and petrol can be more appealing. When buying new in particular petrol appeals because as above it tends to be cheaper for the same power output.
Of course, this all depends on he specific make and model and one should be wary of maintenance on older diesels.