A 40mph is not a default limit and therefore requires a Traffic Regulation Order. The TRO will define where the 40mph limit ends - which may or may not be the same location as the 30mph terminal signs. Historically, terminal signs were often placed "conservatively" - a hundred yards or so before the start of the lower limit (or after the start of the higher limit).
If you were "caught" within 100 yards of the change of limit, it might be worth looking at the TRO (available from the relevant highway authority).
There might also be a TRO for the 30mph limit if it was not always a restricted road (30mph by virtue of having a compliant system of street-lighting).
Whilst 41mph (or even 40.0001mph) in a 40mph limit is technically an offence, it would never be intentionally prosecuted and would be very difficult to prove beyond any reasonable doubt (which is part of the reason why it would never be prosecuted). Approved speed measurement devices typically have a nominal tolerance of +/-1mph, ignoring operation issues. The reason for the ACPO/NPCC enforcement guidelines (10%+2mph) is primarily so that courts can be certain beyond any reasonable doubt that the recorded speed means that the accused was actually exceeding the speed limit. If a police speedometer might over-read by upto 10% and a follow check recorded a speed of 76mph, was the accused exceeding 70mph?
