Bring the discussion back around to the prospect of buying a used car, you do need to consider that there may be a bigger supply of diesel variants in the used market compared to petrol (for the Quashqai+2, Autotrader suggest more than double the number of diesel compared to petrol). What this means is that, engine aside, there is more chance of finding a suitable Diesel than a suitable Petrol and hence the prices you have to pay tend to even out more. Or in simple terms, effectively the fact that diesels normally cost slightly more than the petrol equivalent is negated somewhat by what is effectively slightly more depreciation. So say a diesel with a list price of £22k may end up costing the same in the used market as a Petrol with a list price of £20k. If they were registered before April 2017 they typically have very low tax as well.
The reason I highlight this is that typically diesels don't make sense as a new purchase - especially given the tax changes - unless you do a lot of miles but this isn't necessarily the case in the used market, where I think the mileage threshold is lower.