Hi there,
Thanks for sharing your story and asking for advice.
You may have found that youve fallen foul to a piece of marketing with the 'Last a lifetime' statement: Any well cared for piece of cast iron will last a lifetime.
Regardless of which: the piece of legislation that you are asking about is Section 14 (2) of the Sale of Goods Act, whereby it states that all goods must be of 'Satisfactory Quality' (which makes mention of Durability).
Naturally the durability of a product comes down to many factors including: descriptions, materials, expectations, public statements, finish, price (and so fourth).
Who decides if something is of Satisfactory Quality? Partly you, partly the seller.
If you feel goods do not conform to S.14(2), you are to notify the seller, should they agree to this, the remedies available to you are: a repair , a reduction of purchase price, damages (and in some situations - a replacement [If it makes economical sense to - in your case, it doesn't]).
If I'm perfectly honest, I think the seller is being reasonable by offering you replacement gloss to repaint. The reason I say this is because the paint holds no bearing over the usability of the bench, admittedly, their marketing techniques give the impression that you can just put it down and leave it for a lifetime, but that's marketing for you.
Of course you can disagree with my opinion, and I understand that. If you're adamant that you won't paint it yourself, you can request: them to do it, a reduction of the purchase price or monetary damages.
Adam