When it gets to that time of night, usually around 3-4am, and everyone is seshed up and been really nice to each other. This is also commonly known as 'licking each-others arses'. Best-mated differs from this because you are that drugged up and walking round using phrases like 'you lot actually are my best-mates' or 'just wanted to let you lot know that you are all proper nice people and i love you all'. When you see someone doing this; you can describe them as 'bestmated'.
Mia: "I'm thinking we should cab it to Fox's gig tonight. I want to make it there on time, and I'm SO over the bustration of late nightpublic transit."