unexplained

Hello all, I've had this rather weird thing happen to my car twice now. I've covered around 2K miles and everything seems fine with the car except for two incidents when there was a sudden loss of power. It feels like the throttle is being severely restricted, leading to very slow acceleration...

Picture the scene. Glorious day on Wednesday so head to work in the coupe and, at lunchtime, take a colleague for a very short (10 minute) run down the A5 and back before stopping off at a shop for a sandwich. Come back to car to find it having a small wee in the road which appeared to eminate...