Mineral,
Twas me who posted the picture.
I've been using one of these for over a year now, was given an old one originaly by Reg at WSP to have a look at it which had been suffering from similar problems to what you have been experiencing. I fixed that one, used it for a while the bought my own. I've serviced stack of others as well.
Don't panic, all can be made well again

, I sussed out a fairly easy servicing method for these which can fix the eventual slippage and on the units whcih are under high load (race sails) the occasional jump of ratchet.
We have also found that the White Marlo rope works best with these, I tried lots of different ropes includng a the OEM Chinook rope (quite disappointed there) and even an climbing cord and non compare to the marlo.
I use mine on up to 9m race sails which need a bucket load of downhaul force. I would recommend if using primarily on race sails the the 2 pully version would be a better option as it will reduce the load on the rope.
Back to the fix, it's only about 5-10 mins, although I think in your case the Marlo rope should(?) do the trick.
Myself, I only keep my old extensions in case of emergency.
These extensions in my books are brilliant (not perfect, yet then again nothing is) easier down haul than doing it manually.
I can now also adjust my sail down haul in the water with out having to go to shore which is a advantage at Avalon.
If you want more info drop us a line through the site email and I'll give you more info.
Alby