“Has your mother made a will?” Strike asked.
Bristow looked startled.
“I—yes, I believe so.”
“May I ask who her legatees are?”
“I haven’t seen it,” said Bristow, a little stiffly. “How is this…?”
“It’s all relevant, John. Ten million quid is a hell of a lot of money.”
Bristow seemed to be trying to decide whether or not Strike was being insensitive, or offensive. Finally he said:
“Given that there is no other family, I would imagine that Tony and I are the main beneficiaries. Possibly one or two charities will be remembered; my mother has always been generous to charities. However, as I’m sure you’ll understand,” pink blotches were rising again up Bristow’s thin neck, “I am in no hurry to find out my mother’s last wishes, given what must happen before they are acted upon.”
“Of course not,” said Strike. (The Cuckoo’s Calling, by Robert Galbraith)
Is the bold structure a Whiz-deletion? Or is ‘given’ a preposition to denote ‘considering’? What is the complement for the preposition, upon? For the sentence doesn’t make sense to me, I’m afraid there’s not more to give you what I don’t know about.