GRRR fetch your sh!tron to donnington and we'll see what handles better

thats the VW rant over!

Been there a few times most mornings actually! looking out of the side window front wheels scrabbling on opp lock and just power sliding into the car park at work love it not as good as a rear wheel drive car though

So slidey slidey problems...... your supposed to grin and give full opp lock then blip the throttle and keep it going for sheer grin factor!!!
If you don't like that kinda thing then I'd ignore your brake bias valve and stop braking in corners it's lethal! brake in a straight line and then power out at the apex when you can see the exit outbraking and out cornering people is fun and safer!!!
First I'd be looking down the lines of rear tyres both the same tyres is the tread ok on them, any wearing on either side, tyre pressures ok?
Apparently the golf mk2 is prone to chassis flex causing a slide on cornering whereas some other girder built cars may lift a wheel on hard cornering. you can stiffen it all up by fitting a rear anti roll bar but I have heard it makes them even more tailhappy!
Check your rear shock mounts, bushes and shockers etc as these make such a difference to handling and are a bit of a pain so never get changed.
weight in the boot is a factor as well if possible sub and box central otherwise you get the pendulum effect on corners.
4 wheel alignment may help also front may be contributing to it.
pretty sure one of the above will cure it there's enough to chew on there matey!!!