Thanks guys for all the input.
Sospan I had filled up with fuel in the morning of the previous day from my usual pump and had used a quarter of the tank with no problem.
I am thinking that it is more than a coincidence that the light came on shortly after the object in the road hit the exhaust.
Arranged to have the problem code read tomorrow until then I will not use the car even though I suspect it is not something more serious.
At the same time I can get it on the ramp and have a proper look underneath for damage.
