Sorry to see this Alex. A bad situation but some of the advice you've received so far is sound.
The BAD news
The likely cost of repair (up to £250ish) means that if you decide to take this further legally you can only pursue it in the Small Claims Court. You are perfectly capbable of doing so but CANNOT recover any legal expenses so don't go to a Solicitor. It could never be cost effective.
The GOOD news
The same considerations apply to the dealer in question if it comes to that. IF they will not deal with you constructively when you talk nicely to the Dealerships managing Director (never deal with monkeys: only the organ grinder can change the tune) then you have two courses of action.
1) They should already know you are an active member of SCN, and the local meet scene and you can hint that all the advice you've received SO FAR is that any decent SEAT dealer would be prepared to resolve the situation fairly and sensibly as a purely GOODWILL issue to a loyal customer.
2) That you are fortunate enough to have a source of free legal advice and that for the amount involved you have little choice but to pursue the matter in the Small Claims Court even if it is your word against theirs.
You will be able to produce ample proof that you are extremely proud of your car and careful with it and probably witnesses that it was undamaged on arrival at the garage (otherwise you'd have been asking for aquote to fix the dent) The issue is thus is it more likely the car was damaged whilst in the care of the garage or whilst parked securely on your drive.
More to the point a day off work to attend a hearing should be less costly and less hassle to you than it is for two or three witnesses from the dealership.
MORE GOOD news
Despite the understandable caution of the moderators who obviously don't need the hassle which may be possible (though ill-advised from the dealers point of view); you have NOT libelled anyone YET. You have not named them and even if you had you would have a good chance of establishing that your allegation was true and justified. Again in the real world the dealership could not or would not afford the advice/hassle to pursue you personally. Sorry to say but your not worth it (purely from the financial point of view I hasten to add).
Just stay calm and try to be un-emotional about it. We all know you love your car but this is money, business and law so keep a clear head.
If they haven't agreed to let you meet the MD by say Wednesday let us know and you can seek further
help as to the next step.
Keep smiling this can be sorted out.