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Plus the dealer gets to carry out the work under warranty and claim the costs back from Vauxhall so they its in their interests because it generates income.

No, the dealer won't be doing that!!

This bit of one of your earlier comments implies that, although its hidden from normal customers there is a valid warranty claim for the repair.
"The service manager has been on the Vauxhall warranty and recall site, only available to them and found something. It now turns out that Vauxhall DO admit there is a manufacturing fault and that the lights should be replaced at no cost to the customer if and when they fail!"


So basically the dealer does the work and then reclaims the costs from Vauxhall.
It won't be particularly lucrative for them - VX will have a specified time for doing the job that will be tight - but it will ensure that one of the techs has some work to do for, say, an hour to help fill their day and earn another £50 for the dealership.

If they did that, they are still on the hook for warranty for work carried out, charged or not. Better to pass the buck on to Vauxhall.