In reply to an earlier post on FRA using DTVA because of low operating costs, the main reason that FRA use the airport is due to its ideal location smack bang in the centre of the 3 air defence stations that are situated up and down the east coast, which are it's main customers.
Also, the CAA relocated their flight calibration unit from Stansted to DTVA in the mid 90's which made it an easy acquisition logistically into the FRA group, hence FRA and FPL being at DTVA.
DTVA is usually a very nice place to work from but the lack of ATCO's and the knock on effect of not being able to stay open when we need them is an almighty pain in the ar** leading to some unhappy crews, unhappy FRA management and unhappy FRA customers.