OK well my 0.02cents (as was brought up in Car industry, till folks finally sold garage business late last year).
Bear in mind I have no idea of insurance in UK right now after 6 years in Texas.
Please pm me for more info. if required as could write a book here.
Firstly, Diesel operating costs are indeed (at the moment less) BUT their maintenance costs are NOT. Diesel parts cost alot more than petrol. Glow plugs etc. and fuel system is more complicated (to a point) and more costly.
Japanese cars, on the whole, are the best bet for longevity.
Toyota produces amazing engines and are VERY hardy. The parts are not as cheap as say Nissan but they run well.
Nissan is also very good.
Mitsubishi was more due to less imported vehicles.
Subaru, great cars but expensive to maintain.
Honda, also great cars, little maintenance usually.
European.
Citreon is not a very strong car (was'nt anyway) and their suspension system is very complicated (hydrolastic) on those models.
Renault, not bad car but parts were overpriced.
Fiat, Italians know NOTHING about electrics. Repeat after me, NOTHING :)
VW, well made, run well, not most fuel efficient (largely due to weight , read well built) parts can be costly, not that some models break down much.
Ford may be best bet for maintenance/cost ratio. Parts are readily available and cheap which ofsets other costs.
As I said I could write a book (even after a 6 year absense not a whole lot changes).
Any specific questions feel free to ask. If I don't know I can find out.
Nick