Daz,

I know all these suggestions might be confusing/contradicting, but I would think you might require more castor angle on the front to reduce the nervousness, ie shorten the tension struts to bring the wheels further forward in the arches. This will make the self centreing action of the steering stronger, the downside is that it will also make the steering heavier.

Fitting a rear anti-roll bar will only promote oversteer ie make it turn in even more sharply and hence more nervously.

You should have about 15minutes of toe-in on each of the rear wheels, this promotes straight line stability so check that out too.

Hope this helps,

Martin