they must be selling well enough for the promoter to buy in for more shows

The promoter put 22 dates on reserve with the venue and the same 22 dates on reserve with the band, this means he pays a small amount for the chance to pay full price later for each extra group of shows.

The promoter decides when and if to put shows on sale. If the promoter thinks he can make a profit on three more shows then he just sends the band and venue a check. If he doesn't think he can sell enough to make another show worthwhile it doesn't cost him a penny so sales and profits must be good in London. Plus each extra show is a little sheaper for the promoter as he doesn't have tear down and set up costs, so every show ofter the first one has extra profit built right in.

Dave