Lofty,
Get some front springs, whichever manufacturer you like, i think they are all pretty much the same. Make sure they are NOT -55mm ones, these are far too low and damage other parts far too fast.
Re-setting the torsion bar is easy, as long as you have the right tools. And another pair of hands is good also.
Jack up the back, put axel stands anywhere underneath but the axle (you need this to be free). Whip the wheels of and then the caps that cover the ends of the torsion bars. (There are 4 bars under the car, the two furthest forward are torsion bars, the other 2 are anti-roll bars, ignore these)
Spray lods of WD40 into the end of the bars, into the splines and also do this under the car as well. There is a plasitic clip on each bar that hols the handbrake cable, loosen and remove this on either side.
Now comes the most difficult bit. You will need (ideally) a very large slide hammer or driveshaft puller. I use the latter which is very affective. Get a 10mm (i think) bolt and srew the hammer into the torsion bar, nip it up but dont strip the thread!
Pull/hit/smash the hammer/puller as hard as you can for as long as it takes to ease the splines out of thier housing. This can be very hard if they have never been out.
Once one is out get a long thin bar and slide it where the torsion bar has just been removed fromm to tap the other torsion bar out. I used a 106 rear roll bar. This is perfect. Then knock it out using a sledge hammer.
You must take a meaurement from somewhere on the hub to the wheel arch on each side (should be the same). then you subtract the number of millimeters you want it lowered by (no more than 35 i hope!) and you get your new measurement. Lift the hub on each side to the new measurement and while someone holds it (can use trolley jcak) tap the bar back in. Do this on both sides. Copper grease the splines before putting the bars back in(!)
Replace cable clips under car and also caps on splines and put wheels back on. Lower car to ground. You will be lower but the axle still needs to settle a bit and will sag a few more MMs after a run.
Now the back end should be alot more stable and not so slidey.It will however, make a slight weight transfer from front to back. This can cause front wheels to lift over bumps (not good) so you should put front springs on at the same time.
Your car should now be awesome!
Hope this helps mate. It is very easy to do, like i said, with the right tools.