In reply to The Lemming:
I've just started a project for the summer, taking photos of the plethora of urban horses ('Chav Nags') around the area I live in. It's got a few funny looks when mentioning it, so it definitely seems the right one to do.
As far as trespass goes, it's a civil, not criminal offence, so you can't be removed by the rozzers & won't get a record for it as long as you don't nick anything/smash anything/go there to stop someone going about their lawful business. I've found that when caught somewhere I'm not meant to be, just apologise whilst making your way off-site as quickly as possible. Security guards have no powers over you whatsoever, but that doesn't stop them thinking they do.