Well, MAJOR thanks to Apple and especially to Craig at the Glasgow store Genius Bar. This kind of service is why I continue buying from Apple.
I phoned shortly after making the post above, and made an appointment with the Genius Bar, explaining the situation to the woman about my headphone socket.
When I got to store for my 5:20pm appointment (it's worth pointing out that the store is 5 minutes walk from my work in Glasgow City Centre), I was seen promptly and explained that it may seen trivial, but a crack had developed at my headphone socket.
Craig said right away that I could have it replaced, no problem at all. He did warn me I'd lose my screen protector and Invisible Shield back protector, but I didn't care.
He used the magnifying eyepiece to inspect the crack and said, "Thats only going to get worse". He then checked the dock port and headphone port with the torch enabled magnifying tool for water damage. I wasn't worrying, I knew it would pass that test as I never use it in the rain, etc.
Craig filled in some details on his Macbook and said, "OK, I'll just go get you're replacement". He came back with a refurbished iPhone 3G and before removing my SIM card he activated the wipe function on my (now) old iPhone and let me confirm the erase by pressing the button.
My new iPhone was then activated and a new receipt printed out, the deatils of which I found interesting:
Issue Verified: Yes
Known Liquid Damage: No
Issue: Crack at headphone jack
Steps to Reproduce: Visual
Proposed Resolution: Swap
Cosmetic Condition: Excellent (was proud of this)
Many thanks to Craig, he could not have been more helpful.