Not a big telephone fan. Here's the short version:
Trying to work on the mainsheet in place is pretty much impossible - it just gets too skinny in there too fast. You've got to remove the bolt that anchors the fore block, and as you slide it out, slide a skinny, rigid probe in in its place - a piece of coat hanger might do. Then poke through, alongside the probe that's holding the block in place, a piece of strong, flexible wire. I used an old steering cable. That's your fish tape; once you've secured both ends so it can't disappear inside on you, remove the probe. You can now pull the mainsheet assembly aft and remove it for servicing. Reverse the above procedure to reinstall.
It ain't nuclear physics. But be [i]really[/i] careful to make sure there's [i]no[/i] twist in the mainsheet when you reinstall it.
If you want to talk further, I'm at email@example.com
, or take it to the TF list on rahul.net - I don't look here too often.