Any UK Members give our overseas Friends first go on this Thanks
This is a saying in the UK workplace
I wonder it it has travelled outside the UK?
Ask a guy: "Will You Be Mother?" What is his reaction?

Does he pour the tea?

That is indeed the "magic" sentence MilesW
To some Foreigners it may sound very "risque?"/double entendre?
But all it is a way of asking "Do you mind pouring the Tea out for us?"(quite polite)
The other way of saying it is:
"Will you do the Honours?" (but that applies to many invitations)
Hardly a mention about this on Google