Couple of quotes you might find useful to know about:

Didn't know that - how modern is "modern" though ?

Thames Valley Police: "Inaccurate speedos – the legislation referring to ten per cent accuracy went out years ago, modern cars have speedos which are allowed a tolerance that can only read fast not slow. So if your speedo says 30 you will be doing 30 or less but not faster."

Didn't know either, been a while since I took my driving test
On the back of my NIP: "I didn't see any speed limit signs. How would I know what speed applied?
It is most likely you were in a 30mph area. The Highway Code clearly says that in a built-up area with a system of street lights the speed limit is 30mph, unless it says otherwise. The local authority is responsible for the signs on the highway, but legally cannot put up an extra signs to reinforce the speed limit in a 30mph area".

So, in summary, if you don't know, drive at 30.
I would love to know what constitutes a "built up area", cos I don't think that where I got caught does.
Last, I can't understand why legislation prevents local authorities from putting up extra signs in areas where it is most critical that people stick to the 30mph limit The mind boggles...