Tim,
"Come on, someone out there must know what it's all about?! If it was painted-up like that just for fire/rescue work, then they must have had some very enthusiastic (or bored!) paint finishers at Thorney Island!"
No "M" number was allocated to XM346. If it had flown at Cranwell in that camouflage finish, surely a photo would have come to light by now.
Remembering the RAFM/IWM collections, scattered around various RAF stations, there were plenty of oddly painted a/c, particularly the ME262 at Gaydon and the FW190 at Biggin Hill. Enthusiastic, bored or stocks of expired paint is probably an explaination.
Ciarain.