I did wonder if it could be the diesel - every time I float that thought though, folks tell me it cant be!
I might try a tank from somewhere else next time and see. It was just weird though - I was ticking along on a high % SVO mix, quite easily able to go from 70-80 to overtake on the motorway, pulled off, filled up so was close to 100% diesel and it was an effort to get to 70 after that. 100 miles later I finished that bit of the journey, put 30 litres of veg in and back to being fine again.