Now you've got it at +5, you need a certain ember to get it to +6, and then you can use larger chunks (or a certain colour depending on the upgrade you want) to get it to 10. Then you'll need another certain ember to get it to 11, and then larger chunks to take it to 15.
So established a shortcut now. Made a right meal out of getting the cage key and coming back. I jumped over fine, but coming back to the bonfire i missed directed my jump and died, so respawned, and i went to kill the magic snake dude as per usual for a easy 500souls, and instead of going back up i mindesley went straight on and fell down the shaft (got a nice ring out of it though!) made my way up back up fighting along the way, misdirected my jump twice (but somehow survived - sniper crossbow!). Eventually got my 12 k souls back . Phew.
Anyway. Just brought a couple of longswords. I'm going to direct each one on a different upgrade path and just see what happens. Got a magic sword +5, and a +5 normal. So need a couple more in time.
One thing though, i'm at +5 on my magic longsword, but can't upgrade no more, its not even appearing in my upgrade/reinforce bit to see what i need. i think i got what it is (blue titane chunks?), but can't do no more? Or is that one ready to ascend to a further level?
note that certain paths will stop at 10. I think it's only the 'normal' upgrade that goes to 15.
In fact, this isn't really spoilery, and will help everyone -