It seems that you're only course of action is to negotiate with the dealer. They could not have been reasonably expected to remove wheels to look for part numbers, and unless the wheels were quite definitely different in appearance (neither you or the dealer noted any difference) & this was concealed, there is no deliberate attempt to mislead on their part. The other course of action is to reject the vehicle as not fit for it's intended use, but the time gap to do this has expired, and as you've had it since June, it clearly is quite useable.
However, it is implied that a VW approved used car would come on VW wheels unless stated otherwise. You could say that, as a VW approved used car, you have a right to expect VW wheels, as they are standard fit on VW cars, which is the line I would use. I would also be inclined to up the ante by calling VW Customer Care, they may want to help. As the vehicle has replica wheels, this has a significant impact on it's value; you should, if the dealer doesn't want to do anything else, demand some form of financial rebate to reflect the lower retail value. You may not get 4 new Monza 2's though, they are £550 each.
I think that trading standards may not be too interested, however Citizens Advice may be able to guide you as you go.
Above all, be firm & polite. Don't back down, take notes, keep a record of who you speak to, and you should reach a satisfactory outcome
