Hi Chevvron,
CTE was Central Training Establishment...it may have changed name later on.
It's interesting that you mention another Victor diverted (albeit to Lindholme) in 1972 as friend who worked at Manston at that time thought XL511 had diverted in in 1972 and never left, but that can't be right as '511 was still operational in 1986 having taken part in the Falklands conflict. .
Ed