Anybody else had to do this, have to cancel retirement visa in the morning, fly out to Singapore same day to get B for work visa, as obviously I am going to be working here, cancelling visa and flying out same day sounds like good grounds for a screw up?

If you don't have a re-entry pemit there is no reason to go to immigration and cancel your permisison to stay based on retirement. Leaving the country will automatically do that.

^ I have Multi-entry visa, so it would still be valid...

The embassy/consulate will cancel that when issuing a new visa.

I am not sure if he has a retirement visa or extension of stay with re-entry but believe he should cancel it prior to leaving to be sure. Do not believe any Consulate will be prepared to override an immigration stamp and from what I gather even a Consulate stamp is not something they are keen to cancel (unless they issued it).

I guess I better cancel it, thanks for the help.