"Post office staff have been told not to measure or weigh the parcels."

Obviously this message has not got though to sub-post offices.  I've had an item refused as it was too large and comment made on another that the postage paid was far too much (marked as small parcel and would easily have been large letter). Our postmaster ALWAYS weighs items  whether prepaid or not, and if any kind of letter puts them through the gauge - 1mm too fat and he charges the next level.