Renault Clio getting battery charging light alert. garage says charging is fine but for £400 a new alternator would solve it

Don't know about your car specifically , but if it's charging o.k , on a lot of alternators there is a thin exciter wire that usually puts the light out .

Thanks .... what is exciter and where is it?

It's the name given to a thin wire on alternator wiring / plug . Being honest not really sure , just remember that's what my auto electricians told me was the fault when they completed the job .