Me? I'd probably go down the local trading standards folks, taking all the info I had and get their immediate advice on the legal stuff and what rights you have but don't get them directly involved just yet.
Stuff to take, any evidence you have really, copies of the adverts posted by the dealer relating to the vehicle in question (if bought on line go through your browser history to get a copy and then save it as well as printing it).
Next job would be to speak to the dealer and see what he is willing to do to help (don't tell him you have already taken advice), start nice then ramp up depending on what he says. Look around the car lot at the same time this cars nice, I like that one over there sort of stuff, take the wife... girl friend whatever might even give him the impression you could be in the market for a second car for the good lady if things go well with the fronty, tis all in the negotiations and impression you put over. Get him thinking if I make this guy happy with this fronty he's gonna buy another (don't buy another ffs, but he don't need to know that).
If he is willing to fix it and your otherwise happy with the car then go that route, if he's either not willing to repair or your not happy with car then I think I would go down the 'not fit for purpose' and miss sold (service history) routes using the lack of service history as an indication that the vehicle has either not been serviced or poorly serviced in the past and this has directly led to the issues you are now having with it and as a final nail in the coffin so to speak go back to Trading Standards going down the route that he knew of the possible problems from lack of /poor servicing and tried to hide them by initialy stating that there was a good service history but is now unwilling to back up that earlier statement.
If you know the previous history of the car ie the original dealer that first sold it and where it was based then it might be possible to track down some sort of service history via the original dealer and any other local VX dealers via the VRN.
You might even be able to 'up' the warrenty cover on it, or even extend it into a long term cover type thing. But whatever you don't tell the dealer about the problems before asking if you decide to try this
Good luck anyway.... and do tell us how things worked out.
This posting may have been helpfull, on the other hand probably not... Either way, I may have had drink when posting it in which case it's probably not as helpfull as it could have been
Work safe, and remember all technical problems can be resolved with a hammer, the trick is knowing where to hit it and how big a hammer to use (I often find it may also help to have a spare bit in the shed to replace the bit you have just mullered

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