Looks like Skybet won't list it (which is odd as I've never know a bookie to turn down the chance of making a few quid...) So.....I'll come clean.....the name I was given is Radiohead.
Mate who was working spent Saturday night touring crew bars with some guy he met in Maceo's, who then used his sparkly AAA pass to get him in as a guest to T&C Green Room, Acoustic back stage etc etc....he told him he knew Radiohead were nailed on for 2020 and when questioned he said it was because they were Michael's favourite band. Seems unlikely but there are precedents for returning in 3 years, there are rumours of new material for 2020, they could be the act Emily says haven't been mentioned yet.....and....and they're just epic basically. Perhaps the 2020 lineup is simply going to feature the biggest headliners from the last 50 years.....and they fit nicely.
I'm probably going to get some stick for it but there you go...just passing on what we herd (herd....see what I did there?)