If I read the posts above correctly, then you have owned your tf300 for over 8 months now - I think, in this case, I would not pursue the matter further with Asus - they have not accepted your claim & are extremely unlikely to change their position. I don't think you have any chance of getting Currys to do anything, either, give that period of time. To be honest, neither would I try to claim on your gadget insurance, even if they do agree that the mark is covered (which they may not) - the £50 excess alone would be enough to put me off. I think I would be more likely to sell the tf300 & move on (CeX would give you around £200 for it) - or just put up with the mark for another few months & then move it on.
All the best