Whenever there was a need to refuel at night from another VC10K or TriStar. Such as on night one of Gulf War 1.
During one of trust-me-Tony Bliar's bring-a-bottle Balkan wars, we had to ensure that all squadron captains were capable of refuelling at night. The scenario included several VC10K / VC10K prods, plus maintaining in contact for 10 min including 2 x 180 deg turns in contact. Then some TriStar night jousting.
So quite often, really.