Im not suggesting for a minute that £4k isnt a lot of money (cause it is!) but lets keep this in perspective here...
The alfa garage are probably quoting book times to remove the gearbox and driveshafts etc. Any independant will bill for either a set price based on their experience or by the actual time it takes so it will always be cheaper (plus their labour rates are less), but it doesnt mean the alfa dealer are 'ripping people off'...
Also when an alfa dealer replaces a gearbox they replace the gearbox. They dont do what the independants do and fix the old one - nothing wrong with fixing the old one (which usually involves welding up the very expensive case which is the biggest part of the cost when a diff takes out a gearbox) but compare apples with apples. £70-£80 worth of welding to your case plus the hours labout stripping it is always going to be cheaper than either a brand new complete box or a brand new casing.
Replacing the dodgy diffs with a Q2 is a sound investment though, and it is possible to talk the main alfa dealers into doing it as shown on this thread as the original diff is actually more expensive (list price) than the Q2.
good luck whatever happens, just chalk it up to unfortunately buying from those ******s at veloces
