Thanks to Merlin for this.
I've found a way to open it easily. follow this:-
lay the router down on its side, with the front nearest to you, and the badge (2wire or BT ) to the left of you,if its to the right, turn it over.
If you've got that correct,to read the writing ,power,ethernet,wireless etc,
you should have to tilt your head to the left.
With a thin bladed knife, insert it between the black plastic light guard, and the silver surround
Now the tricky bit, gently prise the black plastic light guard towards you, until you can see two silver tags, while keeping the black plastic bit pulled forward, gently ease the silver surround upwards, until the silver tags clear the black plastic light guard. (If it will not move up, it's because you have not pulled the black plastic forward enough.)
Gently work that opening around the two sides of the router, until the whole of that side comes off
I've just read that through and it looks really complicated, but its not really, just looks it.