Reviving an old thread, eh?
Im in Rugeley, and use Motormech in B'ham regularly - can't recommend their service enough.
At the moment, my 159 has done a few random MPG drops, usually on my commute to/from Derby. Firstly I had a leaking EGR valve which was making my inlet temps soar, and therefore cripple the air/fuel ratio. I replaced the MAF (not recommended, v. expensive!) that didn't do anything. My EGR is now cancelled out of my (Motormech) remap and it's still happening occasionally.
I've never had a DPF warning, but my car occasionally drops to 3 cylinders so I'm thinking I've got a blocked/knackered injector - so I'll be shoving some of that über concentrated injector cleaner through the next fuel tank, before I shell out many £s on a new'un.
RE: local companies, I don't know of any local alfa specialists as I've always trundled to brum and Motormech - if you can stand the 45 minute journey, you WON'T be disappointed.
EDIT: wish my car got up to 50mpg...
DOUBLE EDIT: what specs are your car? mileage, typical journeys etc - Mine does 65 miles/day for reference.