I have seen it stated categorically in Another Place that the existence of ACPO guidelines on speeds for prosecution create a legitimate expectation (in the legal sense) that no speeds under these limits will ever be prosecuted and that any attempted prosecution for speeds under these limits must therefore fail.

Sounds like hooey to me - what about Mr Brunstrom and his strict enforcement policies in North Wales? - but some documentary evidence one way or another would be much appreciated. Thanks.

IIRC this argument was tried in Dr Science's (wife's) case and failed.

The ACPO Speed Enforcement Guidelines say this:
"Driving at any speed over the speed limit is an offence. The differing speed limits are generally related, and proportionate, to the risks to all road users using that road. Where police officers consider that an offence has been committed i.e. that a motorist has driven at any speed over the relevant speed limit, they should consider whether it is appropriate to take enforcement action against the offender.
The police service now uses technology that enables it to prove that an offence has been committed as soon as a driver exceeds the relevant speed limit by a very small margin. Motorists will therefore be at risk of prosecution immediately they exceed any legal speed limit.
The guidance to police officers is that it is anticipated that, other than in the most exceptional circumstances, the issue of fixed penalty notices and summons is likely to be the minimum appropriate enforcement action as soon as the following speeds have been reached:
The Well Known Table Follows..."
IMHO this passage gives no "expectation" legitimate or otherwise that speeds below the speeds in the table will not be prosecuted.
You should read all of the guidance and not just part of it then you would see that what you and others may believe to be a "legitimate expectation" is merely a misinterpretation from an incomplete comprehension of the guidance.
The documentary evidence is in the guidance, have you read it?

But according to ACPO and the CPS the guidance is just that - guidance. It carries no weight at all.

...even less reason to have any sort of expectation then!

Exactly!

The probative value of the ACPO's utterances is about the same as the value to the public of the ACPO itself.

(i.e., Nil).