Thank you that's all reassuring. I was having visions of having to go to court in Northumberland.
If the information about the penalty had been on the letter I might not have wound myself up so far.
Also, I couldn't find something in a cupboard despite it being right there, so momentarily convinced myself I might have dementia. 
I'm happy to do a course and pay a fine. Less happy to have points.
And still a bit confused about how it happened.
Questions you've raised-
Google maps shows it's a 30.
I don't use my satnav for speed as it's v old and not up to date. I use the signs.
The 30 zone starts very near the petrol station I pulled into, which makes it even less likely I was doing 40 there.
But life is too short and stressful to argue it out.
I was surprised I'd been speeding. Extra surprised when my black box appeared to back me up.
I'll wait for the next letter and take it on the chin.
I did wonder about ringing my insurance, but that's a new can of worms isn't it.
I shall channel Elsa and stop fretting.