Another update:
So, I had basically decided to let this one go. I phoned DHL to pick up the guitar, and despite telling me (before I started the dispute) that they would insure a guitar without a hard case, when I phoned up to book the collection they told me they wouldn't.
Grrr.
Anyway, I basically decided to let it slide... my feeling being I'd rather have the guitar and no £350 than not have the guitar OR the £350 if it got smashed in the post...
However today Ebay contacts me, saying the seller has contacted them to resolve the dispute. Apparently now Ebay is going to pay the return postage by Parcelforce (they sent me a prepaid label). Is it just me or does this mean it's their problem if it does turn up smashed? From the sounds of things it sounds like Ebay will refund me the money as long as the tracking number shows it's been delivered.
I mean I'm still paranoid, I'll take a bunch of pictures of the guitar and that day's newspaper on the day before I post it to prove I didn't smash it etc., but yeah.
Any thoughts? Thanks again for your help (and sorry to keep bothering you guys about this) :)