My thoughts are these:

An approved device operated in the approved manner is assumed to be reliable unless the contrary can be proven. If you wish to defend the charge on the basis that the device could not be relied upon to measure your speed the burden falls to you to prove it. Simply saying the tests are only carried out at a shorter distance than you were detected will not cut the mustard. If you get hold of the documentation for the device you will probably find that it is deemed reliable at a far greater distance than 150m – probably up to 1km. It is not sufficient to show that “this could have happened” or “that might have happened”. You have to prove that something definitely did happen in your case which caused unreliability. This is very difficult even with expert assistance and near enough impossible without it.

You will almost certainly be offered a fixed penalty (£100 and 3 points). In fact, that speed is only slightly too fast for the offer of a course (up to 75mph where a limit of 60 is in force).

You would be well advised to accept such an offer. If you challenge the charge and fail (which is highly likely for the reasons I have outlined), the cost is high. You will pay a fine of half a week’s net income, a surcharge of 10% of the fine but most crippling of all are the prosecution costs. These will start at £620 but could be much higher if the prosecution has to enlist expert help to counter yours.

You must return the request for driver’s details within the 28 days allowed. Failure to do so means you commit a separate, more serious offence which carries a much heftier fine, six points and an endorsement code that will cause you insurance grief for up to five years.

If you do accept a fixed penalty do not forget to pay and send in your licence within the time allowed. Quite a few people fail to send in their licence and end up seeing the matter dealt with in court, where it will cost them far more than £100.