Likely culprits are:
1- reset the ignition (lots of of threads on this)
2- Check induction system & air filter
2- Clean or replace the MAF
3- Replace lambda sensor
4- Check coil packs & spark plugs are working correctly (easy on a TS a pig to check the rear bank on a V6)
Good list there
GTVDan - have you scanned/had the car scanned for fault codes?
Lambda sensor, often reported as a fault code, is a tricky one as it's usually a symptom rather than a cause. A split in the corrugated rubber intake pipe, between the MAF and the throttle body, can cause a weak mixture and therefore a Lambda fault. Therefore you really have to take off the pipe to the throttle body and check it properly (it usually splits underneath, can't be seen in place), as suggested by 2-.
You would think that running with a weak mixture would use less fuel, but my experience (with a 166) was precisely the opposite. Because the engine runs worse, the system injects more fuel to try and get the power back!
If there are no fault codes, I vote for the second 2- in the list: the MAF. Especially as the photo suggests the car is a Cup and therefore a V6. MAFs are a common failure on the V6 and if you don't have a spare, it's wise to have one on hand for future troubleshooting. Faulty MAFs can cause two other completely different faults - no idling/difficult starting, or no power over 3000RPM.
It could be any of the other items on the list - and spark plugs/coils should definitely be checked as they are often forgotten - but my hunch is the MAF as I've experienced the same sudden loss of fuel economy on other Alfa V6s where the MAF was the culprit and there were no other symptoms.