RAF Unit In WW2 – F.I.F.D.V. – What Does This Stand For?

Maybe someone here has the answer to this one…

I’m looking through the record of service for an RAF pilot who, after completing a tour with 97 Squadron in November 1942, he gets posted to a unit referenced as F.I.F.D.V., where he remains until posted on to 1660 HCU in November 1943. Under ‘Special Remarks’ it provides the comment ‘Duty with demonstration squadron’. During this period I know that he was instrumental in putting bomber crews through training in evasion of fighter attacks, for which he was posthumously awarded the Air Force Cross. But can anyone identify what F.I.F.D.V. stands for? It comes up on no list of units that I can find.