Bandit - tell us where abouts in Wales your from, and we'll point you in the direction of a good indie. This seems like a bit of a common problem, but your solution is definitely not common.
DO NOT PAY THE DEALER anything like that sum of money. I'm sure you could source the part from a local scrappy for pennies, and a local grease monkey could fit it for you on saturday afternoon on your drive for fifty quid, a bacon sandwich and a few cups of tea.
But to be honest, I think you've been mis-diagnosed. If you let us know where you're from, we'll point you out a good independent Alfa specialist who won't charge the earth and will probably know exactly what the problem is.
Good luck with it.