Well, finally got around to removing the cylinder head yesterday to see what the damage is. Here's my theory on what I reckon happened......
1). Cam chain tensioner stopped tentioning the cam chain for some reason. This is particularly annoying as I only had it replaced recently (by a proper bike garage). Check out the photo - this is how I found it when I removed it.
2). Cam chain flailed around sometimes hitting the rocker cover (some scuff marks inside the cover) and generally slapping around hence the increasingly loud rattle that I got during my trip. Check out the photo of how loose the cam-chain is (this is before I removed the tensioner).
3). At some point, despite taking it very steady, the cam chain must have jumped a cog or two thus messing-up the valve timing and eventually making the valves hit the piston slightly. See photos of underside of head / valves.
4). Exhaust valves bent slightly from making contact with the piston and no longer make a seal and possibly is what made the engine effectively seized. You can actually see daylight through the exhaust port so the exhaust valves are not sealing at all. See photos of valves.
5). On the plus-side, the piston / cylinder look perfect (other than a lot of carbon on the piston). The bore is smooth, not glazed or scored so I reckon that that the piston / rings / bore, etc. are all OK and can be left.
6). Oddly, I seem to have 3 metal head gaskets - is this normal?
7). So I reckon that I need someone to fit / grind some new valves into the head for me, I'll probably need a new cam-chain-tensioner to be safe and then it's just a case or putting it all back together, setting all the timing marks up properly, etc.
8). Incidentally, the cam-shaft looks perfect, no scoring/marks/grooves and all well withing manuafacturer's tolerances according to my micrometer. Similarly the camshaft bearings look fine and the bit where the cam-shaft runs in the head looks good too - again no scoring/marking/grooves, etc.