Sorry to be pedantic, but I cannot see on the Little Gransden site that one should not descend to circuit height deadside.
Lt Gransden flying clubm - EGMJ
It says join downwind, beware of glider site 1 mile to the north and the cct patterns are all south. I would have thought that a glider site 1nm to the north would have made no deadside joins a no brainer. As I say, I'm still a wet behind the ears power guy myself, it's not you I have the issue with but your instructor for not seeing the problem. If I can see it as a novice why can't he.
I've never been to Gransden by the way, didn't even know where it was until I read your post.