Slippy roads in heat

I know the roundabout you're talking about and in my opinion it is more likely to be diesel on the road.

Hot tarmac is likely to become more sticky rather than less. Your tyres will also become more grippy with heat as well. Most likely explanation is as above.

I had a similar experience coming back from Cromer. It was at Peterborough and there was absolutely no reason for the van to slide sideways and for me to have to correct it but we were in roadworks and I am sure that it was down to diesel on the road surface. I was just glad I wasn’t travelling at speed. Just goes to show you that you cannot relax your concentration for a millisecond!!