go to settings/itunes and app stores and sign out
reboot ipad by pressing and holding the home and sleep/wake buttons at the same time until the apple logo appears on the screen, then let go.
go to settings/itunes and app stores and then sign in with new credentials.
Thanks Grumpygator -that was brilliant! I didn't even have to reboot - I was able to change the address directly after signing out on the itunes app.
Sorry, I don't know how to put the green tick in to give you your 10 points