Thanks Betty, I found that one last night
Looks like the axle will move forward 11mm. I'm at work at the moment so I can't measure anything. I've been looking at some photos I have from before and after the new chain went on.
Before - the axle was right at the back of the slot.
After - the axle is now right at the front. Looking at the photos, I reckon I have very little, if any, room to move forward
You'd think the mechanic who put it on, would have at least left a little room in front
So the plan is:
Wait till I get home, and take some measurements. I'll also be able to count the teeth on the rear sprocket to confirm what size it actually is.
Then I'll get a new 15 front, new 41 or 43 rear (whichever I decide on), and a new chain.
My existing chain is only 6 months old, but at least I can keep it, and the old sprockets to use if I ever go touring...
A little more costly than I planned.
I don't have a missus to spend money on, so I might as well buy "jewelery" for the bike instead