(22 Jul 2016, 6:53 pm)R852 PRG wrote DiagTest are based in Unit 5 of the Sunflag Industrial Estate, while Arriva are based in Units 1, 2 and 3. They're the same complex but not actually part of the same depot, although doors connect the two units together.
I think you'll find that DiagTest actually use the same facility as Arriva. since Sapphire vehicles are parked in the same unit as DiagTest on an evening.