I didnt see the diesel to be honest, but the crash did happen whilst leaving the petrol station at the motorway service station
ah, then it's quite possible the diesel was actually on your rear tyres from the service station then rather than on the road...
in 37 years of driving and around a million miles I have seen diesel on the road plenty of times, when you ride a motorbike (as I have for well over 20 years as well as cars and trucks) you learn to spot the rainbow effect on the tarmac and can even smell it sometimes

it's usually on the exits of petrol stations or on roundabout frequented by buses and lorries (in the past I've often seen fuel sloshing out of filler caps of overfilled lorries)
I've driven lots of front and rear wheel drive cars and occasionally 'have a play' when I can feel the surface is slippery on roundabouts and never yet lost it but then I've done several skidpan courses and trackdays so I know what it feels like and how to deal with it - first skidpan was done in 1973 at Brands Hatch as part of the racing school in a mk1 Escort on bald tyres pumped to 60psi and a course covered in water and diesel that you'd struggle to walk across
Tom, I'm sorry you've pranged the car but as said above, loads of these cars are Cat D and at least you know it so better to get it repaired and keep it than get the money and buy an 'unknown'. Get on a Carlimits day or just find a local skidpan course and learn how to deal with the situation in the future