Perhaps at times like these you may need to reminded that there have still been occasions when NCL has been used for diversions when MAN has been fully utilised for that role; BA used to have a policy of restricting the number of aircraft it had divert into 1 station (or so it would seem!).
It's not so much a case of MAN "failing" as being a victim of its own success in having many more aircraft on it's own services than NCL does which means that the staff at MAN need to prioritise them rather accept aircraft on diversion.