So, grabbed handbook and tools and set about adjusting offending item. Found the adjusty things at the back of the swinging thing... then spent an hour looking for me allen keys.
Got the allen keys... and tried to adjust things. However, on closer inspection it appeared that the markers were both right at the front... and the keys wouldn't turn either. Reaction, bugger... new chain required.
Next day, down to the local garage to book the service and get a new chain. Being smart, I thought I'd just double check the chain thing so says to grubby oik "now, this chain thing. I've tried to adjust it but assume that, as the marks are there, it's out of adjusting and needs changing? I've tried turning the screws and nothing's happening."
Oik replies.. "No sir, you see that's a virtually new chain and the markers are at the start of their guage. Did you think to loosen slightly the nuts that fix the wheel to the swining arm at all?".
Me... "err, no
Bastard. Every day I think I'm getting bloody dumber