It's not like you are filming a movie there. If you weren't using a tripod, I'd say just go for it.
I'm in the US, and we have public access television here. People tell me all the time I am supposed to have filming permits and the like, but I just tell them it is for Public access and they seem to go away. I've never been thrown out of anywhere for filming with a lower end looking camera. Even with a tripod. I have had people ask me questions.
I think the liability stuff is more if you are going to be bring in crew and heavy equipment.
Call the groundskeeper and tell him what you are doing, and tell him you are just going to be filming with a small camera and tripod. I bet they'd be fine with that?
If not, bring a consumer grade camera and get the footage that way. A La Michael Moore.
BTW: I'm no lawyer, so you should check with a lawyer before taking any of my advice...