Thanks for the comments. I had the tyres changed, rebalanced and tracked yesterday and it's still there.
It violently shakes the car around when in a high gear(60+) rather than rumbling/humming. If I am in 5th/6th cruising at low rev's, somewhere around 2000rpm, and put my foot to the floor to over something in my lane or accelerate for whatever reason, the car shakes very badly - enough for me to think that I'd rather sit behind the car infront than overtake. I took a look on the Alfa156 forum and that pretty much describes the DMF going wrong, but I suppose it could be the diff, wheel bearing or all of them!!
The car is at 100k miles now, so I suppose it's possible that it's the DMF. I might get the wheel bearings change too, but that'll probably be another £150 a side I'd guess.
As I'm thinking of getting a re-map anyway, I was going to stick an uprated clutch in(if the CG ones from ebay fit the the 20V m?!), so I might as well do that, replace the flywheel and see what happens. The problem is that'll cost me a grand and it still might not be fixed.
Are there ways of eliminated each problem one at a time by testing?! How would you know if the diff is going/gone for example, I haven't a clue?! The garage said something about a cup that the driveshaft sits in at the gearbox end, but if that's worn you normally get shiney spots or something and there isn't any on mine. They also thought that if it the problem was connected to the driveshaft they replaced(diff or wheel bearing) then replacing of the shaft should improve things, if only slightly as it's all connected, which it unfortunately hasn't.
The garage also said they could see the car vibrating on the ramp from underneath when they checked the engine/gearbox mounts, so they thought it would might be more likely to be something gearbox/diff/flywheel relating due to vibrations being visable when the car is static rather than the driveshafts as they aren't moving when stationary.