G-CEMS = Casualty Experiment Maximum Speed
I think that's the fold back blades being trialled at Elvington earlier in the year. I'm surprised a pic has been posted as it was supposed to be hush hush and a little earner for the YAA.
The system will enable UK air ambulances operating the MD902 to get to the casualty and then onward to the hospital a lot faster than standard model variants. A throw-off from the Military LUH bid made by HD Helicopters under the 'Black Operations' request.
The extended tube at the back produces the thrust by 'reheat' and the folded blades form 'wings' to give the lift. This particular ship doesn't appear to have the extra fuel tank needed, unless this was taken after the internal option was looked at later in the trials.
This pic was taken during the transition stage of flight, when the blades begin to fold. The car to the right of pic, is gathering all the research telemetry, and of course films the process 'live'.