There are legal dB limits in relation to public events (and they’re pretty ridiculous these days – they made some new laws recently that had most of the sound engineering community swearing)… However, if it’s a private party those do not apply, so you can happily ignore them…
As for shutting you down, the police can do that if they get complaints, and are of the opinion you are causing a nuisance (something seriously open to interpretation). Having permission to use the land is very helpful, and another helpful thing to do would be letting any neighbours who are likely to hear it know what’s going on (so they are then less likely to phone the OB)… Tell them you’re wanting to have a party, ask them if they mind, and as a best case scenario invite them along…
The OB do have the right to come onto the land without a warrant, but not to search it… A sec 63 is basically what happens when they come and tell you that you have to stop – at that point you have been sec 63’d (ie asked to stop – if you have permission to be on the land it doesn’t mean you have to disperse, but you do have to stop making noise.)
Basically all common sense – make sure you have someone who is happy to deal with the OB (the landowner would be favourite), who can go and talk to them when they arrive (I got to deal with the OB 4 times at one of our parties this year :crazy_diz. But they were fairly sound, and were more amused by it all than anything…) There’s nothing they hate more than arriving and getting nowt but blank looks and dumb people, and they will be much more likely to shut you down if this happens…
