@Brian ... which remind me ... when the BT engineer came before he started going on about not needing the ring wire connected, in amongst all his mutterings - I'll have to double check see if he's fitted that back or not (apparently my attempt at wiring didn't come up to scratch as it left little "tails" on the wires

so he redid it all).
@sam - its easy enough - I was a complete novice too (see above!) but as long as you make a note of which slot numbers the current wires are in, you can just transfer them like for like.
As I was going to be using an extension socket rather than the main one to plug in my router, I also needed to isolate the wires from the extension socket and, instead of transferring those like for like (ie, 2 to 2, etc), those had to be transferred to a seperate set of slots called A & B - it explains in the leaflet which goes where (wires from 2 in the old slot go to "B", and 5 to "A").
Basically A & B bypass the filter inbuilt into the faceplate, so that you don't end up "double filtering" at the extension end.
Hmmm ... question, @Brian - not that I use any other extensions, but I do have another extension upstairs - does this mean that all those wires in the numbered slots are filtered? Obviously the master socket is, but does this negate the need for an additional filter on by bedroom extension?