A phone extension only? Now I'm getting confused. I wouldn't need a 'phone' extension - I already got those. But I understand that with fibre the router/modem must be connected to the master socket so I would need a data extension. But you say a Data Extension Kit is just a dedicated phone extension.
I'm sorry, I seem to have misunderstood all I've learned today.

It extends the VDSL signal from the master to the new location. That is before the modem converts the VDSL signals into usable Ethernet format.

Ah, I see. So I'll just get a sufficient length of CAT5e ethernet cable, plug one end into the master socket, take the other end to my office and connect it to the fibre socket on the router/modem and take from there?

Well, that is bring out a cable from the master socket to a RJ45 socket & plug the patch lead into that.
Thanks for your help.

The router/modem doesn't have to be connected to the master socket when connected to Fibre. It will work perfectly well on a properly installed phone extension.

No, I guess all that's needed is to run a ethernet cable from the master box to my preferred location of the modem/router.