Just an update on outcome for this. Might be useful in case someone else is searching and having the same issues.
The crank pulley on the v6 turned out to be not a big solid lump of steel as I thought.. It not only has a rubber layer in it for absorbing vibrations but is responsible for balancing the crank and therefore engine!
When I took the pulley off it was clear that the rubber section was starting to come apart. In fact it was running so out of round it had start to wear away the aluminium timing belt cover behind it, which also accounted for all the noise..
Car now has a shiny new pulley and is running waaay smoother than it was. It is even idling much smoother and more evenly with less vibration than before.
Apparently this issue with the can be brought about by having the car sitting for long periods of time.. according to some opinions the belt tensioner is so strong that left in one place the pulley can distort over time. I've ever done this so could to be due to a previous owner.
Another very good reason to drive our Alfas regularly!