Many thanks all.
I am indeed in receipt of a conditional offer having confirmed I was the driver.
I naively thought I may have a route to appeal short of going through the courts, but with a baby on the way and a house move I have no desire to do that so I better accept it.
It's a sickener as it is in my view a very minor offence with mitigating circumstances but I could live with having 6 points. I guess when you hear of drivers going through the courts it's when they are about to lose their licence as it's the only context in which it's worth it.
The whole point of a fixed penalty offer is to provide anyone accepting his guilt with a simple solution, avoiding court, and thus saving all parties considerable costs. It therefore does not cater for arguments or appeals.
Whether it was a "very minor" infraction is clearly a matter of opinion. You were 20% over the limit, and the courts' guidelines suggest a fine of half a weeks earnings.