I'm very wary of cleaner discs as I've known of some laser heads being damaged. The drive plays absolutely perfectly.

The error message I'm getting is: "The disc can't be burned because the device failed to calibrate the laser power level for this media".

This error message has been discussed on Apple forums for the past 2+ years - with all sorts of advice ranging from blowing compressed air from a can into the drive to resetting the International language settings in System Preferences (!) to raising the laptop up so that heat dissipates to running a cleaning disc...

I've wanted an external burner for some time. When it arrives tomorrow I'll get it into action and only then - as you suggest - consider a disc cleaner for the faulty main DVD/CD drive.