I'm sure this will make you all laugh.
Turns out that in Liverpool they really don't like signposting speed limits very well. It was one of those wide suburban dual carriageways with trees that are usually 40mph.. and this one was 40mph but had apparently changed to a 30mph, dark, unfamilar road, late night, long journey.
I considered going back over it on Streetview and looking for repeater signs or lack of, but I suspect the street light spacing would probably spoil that idea.
£60 for pleading guilty isn't it?