What kettledrum gives is the 'industry standard' for artists' names in cataloguing and offers for sale in auction. It is what the trade understands anyway, and always has, but the condition avoids doubt and informs those who are not trade or experienced buyers.
How was the lot catalogued? If the full name of the artist was given in the catalogue and without more ('after', 'studio of', 'follower of' etc) then the picture is represented as being by the artist. You have contacted the auction house. It will have conditions relating to forgeries sold as genuine and the reimbursement of buyers. The auctioneers are liable in the first instance; they are the ones who made the representation to you. The vendor may very well say, truthfully, that he relied on the auctioneers to correctly attribute the work, but even if he represented it as genuine to them that does not absolve them for their failure to see that it was not and any consequent loss. If the auctioneers still say it is genuine you'll have to sue them, pitting your experts against them and their experts. The Antiques Trade Gazette occasionally reports on such actions. These do not make happy reading. The costs involved can be enormous