If there is a fault with the speedo, presumably, that still wouldn't stop them (DVLA) from giving me 3 points and a fine?
If your speedo is faulty then no, it won't stop DVLA from issuing the points and fine
OP.
It won't stop the fine being issued because how would they know your speedo was faulty?
You can put this forward as mitigation to a magistrate but you would have to show, by expert evidence, that your speedometer was faulty. You would also have to show that you were unaware of the fact…speedometers must, by law, over-read, by the way.
As others have said, it is quite possible that the camera has been set incorrectly and that the photograph will prove your speed.
For your delectation, 36(2)
http://www.legislation.gov.uk/uksi/1986/1078/regulation/36/made
and this
The Motor Vehicles (Approval) Regulations 1996 where Section 19
"1. The vehicle shall be fitted with a speedometer capable of indicating speed in mph at all speeds mph. up the maximum design speed of the vehicle.
2. For all true speeds up to the maximum design speed of the vehicle,
the true speed shall not exceed the indicated speed.
3. For all true speeds of between 25 mph and 70 mph (or the maximum design speed if lower), the difference between the indicated speed and the true speed shall not exceed—
V / 10 + 6.25mph
where
V = the true speed of the vehicle in mph."
In relation to the fault, "A vehicle's speedometer shall at all times it is used on a road be maintained in good working order. The only exceptions to this are when:
the speedometer became defective during the journey being undertaken, or
steps have been taken to have the defect remedied by replacement or repair as soon as possible."
In essence, it is your shout. If you believe that the speedo may be faulty and do nothing about it in a reasonable time frame, waiting to see if a ticket arrives, the magistrates may not look kindly upon your defence if you then contest the ticket, if that makes sense. Hope this helps.
EDIT: The simplest thing to do in the meantime is to arrange you follow someone at pre-arranged speeds (say at 20, then 30, 40, 50) and note your readings against the prearranged; they should be fairly close and will be a decent indicator.