Daz, it sounds like you have too much front camber, its great for the twistys but in a straight line at speed the car will be very twitchy.

Dial out some of the camber on the front and if you want the car to still turn in as quick add a little camber to the rear.

what have you got it all set to at the moment, if its all over the place the best place to start would be to set everything to standard settings and just tweak them till you achieve a decent ride. Straight out the box settings for the S1 is not that bad, its the torque steer that kills it, which can be dialled out reasonably easily. Once you start making major changes to teh cars settings, if its wrong it seems to take ages to get it back to being a nice drive.