Are/were there any symptoms apart from the light? Mine keeps coming on and then going off, dealer claims ECU isn't logging the fault so they can't tell what it is. Advised me to wait until the service is due and they'll have a 'proper look'
Trust me, it's not being serviced at the dealer!
I had a similar dilemma to yours when the belt snapped in my GT. It was just out of warranty, (went at 60k, service schedule said change at 72k) Alfa said they would look at contributing, but were very vague about how much of the £2600 they were going to charge I would have to fork out.
Independent charged me £700 in the end, including some other bits, and I've used them (and avoided dealers other than warranty work since)