I have a new 2.2 Diamond - had it 4-5 weeks.
I got a towbar fitted by a local mobile fitter. ( He was good - but not cheap )
Cost me £400 but compared to what Vauxhalls wanted to charge this was about 2/3 of the price.
I went for a Whitter with removable tow ball as I didn't want to spoil the appearance of my nice new motor. The guy also fitted 13 pin electrics for me and this folds up out of the way so it cannot be seen in normal use.
The fitter also informed me that to ensure that the warranty was not void he had to fit a separate loom for the electrics. Something worth asking about when you get yours done.