Alternator trouble

Tried to do the alternator on the Jaguar today, what a pain, took ages to get it off. To make matters worse, when I took the alternator to get an exchange unit, they told me there had been a problem with the alternators on these jags and they had been modified to resolve the problem! They didn't have an exchange unit which had been modified but are trying to get one. Luckily the customer isn't in a hurry.