Presume you've contacted a solicitor by now. Anyway, I would add for what it's worth, that the signing of the delivery note is in my view, exactly that. It cannot possibly be legally binding against you in checking out the vehicle in every way.
For example, it doesn't invalidate your warranty does it? If there is an engine problem BMW GB should be responsible and ensure that a dealer is authorised to remedy the defect. Now, juts because the engine defect is harder to spot at first than the paint damage isn't critical here in my view.
If I bought a t-shirt from the shop, and later discovered it had a bad pull in it, then whilst caveat emptor (literally - buyer beware) does to some extent apply, I would expect them to replace it without a question.
Now the dealer has no evidence to suggest that you have damaged the car and you reported it to them within only 4 days. You only recognised the defect upon close inspection whilst washing the car, so you certainly cannot be expected to have spotted the defect upon delivery.
You paid for a brand new car and you are entitled to a brand new car, free of defect. Any good solicitor knows this - I do. So what I am saying is that you do have a reasonable claim against BMW and they in turn may have a claim against the supplier. Your claim would be best to remain in contract against BMW as opposed to a tort claim against the supplier - in which case the relationship between them and you is less clear and a District Judge may be less sympathetic to you than he would is the claim was brought by you against BMW GB.
So, solicitor will have told you that you do have a claim here. They will then have advised you that they can write to BMW GB on your behalf requiring their agreement to remedy the defect, failing which legal action WILL follow. The solicitor would take your further instructions as a consequence of the response from BMW GB. Am I on the right lines here ;-)