Paul__B:
I am a trifle surprised bperrybap has not directed you to his "i2cLCDguesser" sketch which is what you run to determine the "descriptor" for your particular backpack.
I'm actually trying to stay low for a bit
as I'm working a new LCD library that should make all these types of issues go away.
I really feel for the less technical users trying to get their i2c LCDs up
and working and I'll have solution that makes it really easy,
but it isn't quite there yet.
I'm still working on it.
--- bill