Yep, check your council.

Some have lots of restrictions and rules, and you have to book and pay and sign stuff and get insurance and whatever.

Other councils seem to be much more relaxed.

An acquaintance was going to have a party at a local park until she read what she had to agree to, and the list of things she couldn't do (couldn't make any holes in the ground (so couldn't use a mini marquee etc), consequences if there was any litter, etc.

I'm not sure if she had to get approval based on how many people they were expecting though ... ? Because surely if it was a small gathering you couldn't need to book (eg: if you were having playgroup there, you wouldn't book!).

Just contact the council, and find out.