I see most have added a pulley to the outhaul, with a hook through the eye on the sail. Any problem with the 'one design', as the majority seem to have added purchase?
 
If not, which one should I buy? 
 
Also what length of string should I use on the tack now no-one is using the shackle? 

Hi Mostyn, the outhaul tweak dose look nice, I like the idea of just hooking the clew on, I guess with the addition of a pulley giving you extra purchase it would be out of class.

Not sure where the extra purchase comes from. The standard spec takes the outhaul line thru the clew cringle, thru the hole in the back of the boom and tie a stopper knot. A hook and block does not add any purchase to this, just makes rigging a second  faster.
 
It occurs to me that I rarely actually adjust the outhaul, to the extent I'm wondering whether to take my hook and block off! 
 
The standard Harken hook-block can fall off as the hook is only really a 110 degree bend. The Holt one is a proper hook. Attaching an independent hook to a block tends to give a combined fitting that is a bit longer, which is fine on the 8.4, but too long for the longer footed 10.2 sail.
 
The other issue to watch on the outhaul control is the cleat. I had to change the outhaul line on my old boat to get the thing to actually grip.
 
 

Thanks everyone, I think I will stick as I am, but go to a thicker bit of string as mine slips on the RS supplied stuff.
 
What about the tack line length?
 
Mostyn 

I keep the tack of the spin as close as I can to the bowsprit

 Hi,
I am new to the 100 fleet and was wondering if anyone new how to get rid of the great big crease at the bottom front of the sail by the GNAV bars.
 

Simply change the tack-shackle for a rope one. I started as in the pic here http://clubsailor.co.uk/wp/bimbles/rs100-bimbles/
But now just use one from the eye on the boom. Loop is maybe just over 4" long in total, so 2" gap from eye to cringle. Job done