My understanding of the DSRs, though I;m sure the FE has more definitive information than I, is that you may return any product within 7 days, simply as "unwanted" and you are entitled to the full purchase price.
In addition they can only charge you postage if this is made clear before the purchase.
HondaMan you are correct in that I cannot PROVE that I didn't open the item but, by the same token, can they PROVE that they didn't send it out in that condition? As I didn't look any further than the first one, I don't know what the state of the others was when I received them. I only know that I returned them in the same condition as when received.
Unfortunately this is a case of one person's word against another's. The responsibility for checking the condition of goods received rests with you, the purchaser, and if you didn't do that (and you admit that you didn't) you can hardly make any assertions about how they were packed by the supplier.
My advice is to forget about this and get on with life - you're out of pocket, but not by a large amount. Next time it would be an idea to check the packing very carefully when goods are received. I find it difficult to imagine why a supplier would need to lie about the packing for one item out of four when making a refund.
These days with the competition for business among delivery firms there is no time to check the condition of anything before having the electronic pad shoved under your nose usually with an abrupt "sign in the box mate". Usually the screen is so scratched and obscured there is no chance to tick the box, even if it exists, which says not inspected!
I don't blame you, I would feel exactly the same in the circumstances. Checking with the manufacturer doesn't sound to me as if it will produce the result you want, but you never know. I wish you luck, because the outcome is going to depend on that - there's nothing else you can do.