[spoilers] Lucas. Just Lucas.

[Tea Lady]

I think you have covered most things there A Cousin apart from the past love affair with Maya. This seems real.

(Yet) another thought could be that both John and Vaughn were working for Chinese intelligence. Hence Vaughn having a record at 5.
Perhaps Vaughn and John were double agents though and were secretly working for MI5.

The Chinese recently managed to track Vaughn down again and blackmailed him to get Albany for them.

John faked his own death in an operation 15 years ago, and took another identity with Harry's help i.e Lucas North.

The Chinese now know that John didn't die because either Vaughn tipped them off or they have been watching him and have come back for revenge.

Vaughn though, did some sort of double-double cross 15 years ago, which is why Vaughn is scared of Harry and disappeared 15 years ago.

Harry took Lucas North back to England and he started working for him.