May be being stupid here but I can't figure out how I play a guild match if I go to match and guild challenge I can see all the teams in the other game and play a friendly against them but can't choose guild challenge.

Same here not managed to figure it out yet

First thing, have you been selected by your guild to play?
You can check this under Match -> Guild Challenge -> My Guild

If the answer is no then you can't play.
If the answer is yes then check the following:
Under Match -> Guild Challenge at the top below the "vs" it should say "Preperation Day - hh:mm:ss", that will mean that the guild challenge is still in preperation (it takes 24 hour before you can actually start playing).
Once that 24 hours has elapsed you can then go to Match -> Guild Challenge -> Opponents and there will be a green box that says "Challenge" if you have been selected to play. Click on challenge and away you go