"There would be a possibility of conflict if traffic cruising above the transition altitude were on Regional QNH and aircraft only slightly above it were on the 1013 reference datum. For this reason there is a layer above the transition altitude at which cruising flight should not occur, to ensure satisfactory seperation of at least 500ft."
^^ an extract from the air pilots manual.
And what exactly is the "air pilots manual" please?
Ok I presume this refers to Trevor Thom?
Whilst I have every respect for Thom's work not everything you read in a book is correct! It is surely what is specified in legislation and/or the state AIP which is the definitive guide.