Am I right in thinking that you cannot immediately play in a gvg if you transfer to another guild....if so how long does the unavailability last

You are indeed right. You can be selected in the guild challenge but you cannot challenge an opponent(you can however be challenged) for a total of 48 hours from joining the guild, basically one guild challenge .

However if your 48 hours runs out before the guild challenge is over you can still play in the time left. This was brought into effect because in some of the other games guilds with alliances used to jump players around to get easier opponents and ensure wins.

Many thanks

No problem