There is a bit of scaremongering here. Formations operate within CAS day-in day-out and the overwhelming majority cause no problems at all. The incident quoted by mancboy was indeed very scary but, to be honest, about the only thing that associated those two aircraft with each other was that they had previously been a formation and the so-called leader had managed to engineer himself a complete 'goat'! Furthermore, neither aircraft was ever cleared into CAS (either as a singleton or a formation) so I think it is a bit disingenuous trying to link an airspace infringement and subsequent airprox with a formation callsign. Within the UK, there is a common thread regarding the nationality of formations that get it wrong.