Dave, Steve, I agree with you that if the shop has made a mistake they don't have to sell at that price if the contract has not been made. I was answering Dave's point that a contract has to be in place before the Price Marking Order applies (as I understood him to be saying at 01 Jul 2017 4.:55 AM):

The price marking order 2004, the unfair trading regulations 2008 and the consumer rights act 2015 all seem to cover misleading pricing but only come into force after a contract has been made.

Of course, shops cannot discriminate against people by choosing who they will deal with (Equality Act 2010), but that is a different matter from an isolated genuine mistake over pricing!

On a slightly different point which Mr Red touched on, following a Which? consumer group super-complaint into multi-buy offers Asda in particular changed their practices.

Super-complaint

LFF