Posting on behalf of OH as he's not into using the net!

Collecting our new Doblo 1.9 Dynamic tommorrow (yipee). Having to say goodbye to our trusty Vectra estate after 7 years faithful service.

We need to have a towbar fitted - OH is wondering if it will invalidate the guarantee if we have it fitted by anyone other than Fiat (which will probably cost twice as much). Any advice please.

If a future fault can be attributed to a non-Fiat towbar then yes but lets face it its highly unlikely. More chance of causing a fault by poor/incorrect wiring than the actual bar itself.

There is a good guide to fitting towbars on here somewhere

As has been said, the towbar isn't the problem, it's the wiring that you need to think about, however, follow the guide & wiring is a doddle, everything's scotchlocked in.