Hi there Phil!
Nice choice of motor there my friend! Which engine did you go for? Its great to see another Alfa driver towing a caravan!
I tow my 2005 Lunar Clubman with my Alfa 147 JTD 16V and shes absolutly flawless - never been so stunned at how capable a towcar can be before until I got her!!
Back to the mirrors, well I can't help you too much with the 159 as my 147 mirrors are more curvy than yours. However, I use the Milenco Aero mirror (only the offside) and it clamps on to my curved mirrors lovely - its even the exact same style as the mirror on the car!!!
You should find that this will be good on yours too as it uses simple screw clamps - just don't tighten them up too much as they will break!
If you take your car into a dealer car park and ask them if you can try them on, I'm sure that they will be obliging.
What part of the country are you from?