Ah, hold on. I think I've got the hang of this.
I would need to use a "White Modular Plate (1 or 2 Grid Spaces)", into which I would fit a "Telephone Extension Module IDC - White" and a "25mm Single Euro Module Blank - White" beside it.
The "White Modular Plate (1 or 2 Grid Spaces)" is the same size as a Telephone extension plate, yes?
Depends on the plate, there's two sizes. 68mm, which looks amusingly small (and is usually surface mounted), and 85mm, which is the same as a normal socket.