Thanks to Rob G3XFD for passing on a letter from Alan Betts of Ofcom regarding the sale and use of converted PMR gear. It is a very detailed reply, covering EU directives, Trading Standards, Interface Requirements and the like: the full text may be viewed at www.70mhz.org/ofcom.rtf
According to Alan, any form of modification (which includes any form of reprogramming) of the original PMR equipment makes it a new radio. Intermediate and Full licensed amateurs can make their own equipment for personal use which does cover such modifications, but commercial sale or use other than by Intermediate or Full amateurs would probably constitute an offence, either of unlicensed use (by an M3) or of trading standards regulations.
However, it is possible for a competent body (having filled in all the forms and jumped through all the hoops) to issue a Declaration of Compliance for the equipment for use in the relevant Amateur band. The text and date of the Declaration provided by the equipment vendor should make this clear.
If any Foundation Licence holder is not sure of the status of their ex-PMR gar, perhaps they should put it to one side until they have got their Intermediate Licence?