Disclaimer: AS always, this is a guide only and I accept no responsibility for any damage or injury that maybe sustained through following these guidelines.
Tools required (excluding wheel removal):
1. Breaker Bar (only required if changing discs)
2. 1/2 inch drive rachet (optional)
3. Large screwdriver or lever
4. 17mm open ended spanner
5. 14mm ring spanner
6. Copper grease
7. Piston retraction tool (g-clamp or anything similar for pushing the pistons back).
8. 17mm socket (not pictured)
OK, now down to the nitty gritty. I will do the whole thing including wheel removal just incase someone doesn't know how to do it.
1. Loosen (DO NOT REMOVE) wheel nuts before jacking up the car.
2. Open the bonnet and remove brake fluid resevoir cap
3. Jack up the car and support on axle stands.
4. Remove wheel nuts and remove wheel to expose the brakes
5. At this point it is easier for access if you turn the wheel, although this is not neccesary but made phtographing easier too. You now need to undo and remove the bolts on the slide pins at the top and botton of the caliper. you will need a 14mm spanner on the bolt on the rear, and a 17mm open ended spanner to hold the slide pin to stop it turning. You may not need the 17mm spanner, it depends on how free the bolt unscrews. You don't want the rubber boots twisting too much so have the 17mm available just in case. You can just remove the bottom bolt and pivot the whole caliper up around the top pin, because I am going to remove the disc too I will remove the caliper completely.
top.
bottom
6. You now need to push the pistons back slightly so that the calipers come off easier. I use a large screwdriver through the piston and into one of the disc vents. Lever towards yourself to push the pistons back slightly.
7. Now remove the caliper
8. Support the caliper (I use an axle stand).
9. Now the pads can be removed. Take off the two springs that span across the disc between the pads. And pull the pads out from the front and rear.
10. Push back the pistons. I use a retraction tool but you can use alsorts, g-clamps, bits of wood, whatever. This is where removing the resevoir cap in step two comes into place. Keep an eye on the fluid level as you retract the pistons. If the resevoir is going to overflow, draw off some of the fluid from the resevoir (i use a syringe for this). If the fluid has not been topped off as the pads wear, you shouldn't get any overflow.
11. Now We can go about removing the disc. Obviously if you are only changing pads, you can skip the next few steps. Using a 17mm socket and the breaker bar undo and remove the caliper mount bracket. These bolts are darn tight, so although the breaker bar is not mandatory, it makes the job a whole lot easier 
12. Remove the mount
13. The discs are not fastened to the hub (not on the GS anyway) so the disc will come straight off.
Re-assembly is the reverse of the removal with the exception of using the copper grease in various places. This is not mandatory but it will help reduce/illiminate brake squeel. Apply a small amount between the disc and hub surfaces. Also apply to the piston faces and bwteen the pads/shims/calipers. Make sure you do not get any grease on the pad surfaces.
Don't forget to put the springs back on tht link the pads.
Slide pin bolt torque is 34 Nm (25 ft lbs)
Caliper mount to hub bolts are 118 Nm (87 ft lbs)
Once everything is back together and the wheel goes back on, tighten the wheel nuts in the sequence shown below, torqued to 103 Nm (76 ft lbs)
I hope this tutorial helps somebody. Any questions, please feel free to ask.