I performed this mod myself today and (after a nudge by ColonelK) thought I should share the joy!
In order to perform this upgrade you will need:
1 pair of 286mm disks from an Omega
1 pair of calipers from same
1 set of brake pads to fit
Extra brake fluid
Tools:
5mm Allen key
7mm Allen key
18mm Socket spanner
10mm Spanner
11mm Spanner
Molegrips
First, obviously remove the front wheels and support the car on axle stands.
Using the molegrips, crimp off the brake line as close to the caliper as possible. It is wise to use a piece of card placed around the brake line to prevent damaging it.
Unscrew the line from the caliper using the 10mm spanner. With the line crimped there should'nt be too much leakage from the line but be prepared for some fluid to leak from the caliper.
Now to remove the caliper.
Remove the caps from their rubber seats behind the caliper (there are 2 - top and bottom) to reveal a 7mm hex bolt. These bolts secure the body of the caliper to it's bracket. Undo these bolts using the 7mm allen key, and remove the caliper and pads.
You should now be able to see 2 18mm bolts on the back of the caliper bracket. Remove these and the bracket.
Remove the disk by undoing the 5mm hex bolt and gently tapping the disk from behind.
Now fit the new disk - you should be able to use the original 5mm hex bolt to secure the disk. Mine is a 1992 2.0 and it fit perfectly.
Now fit the bracket for the new caliper using your original 18mm bolts. A bit of thread-locking compound would be recommended at this stage.
Fit the new pads into your new caliper - you may have to press the piston into its piston a little to do this and have sufficient clearance for the disk. Make sure you have the correct caliper for the side of the car you are working on - the bleed nipple should be at the top when fitted.
Mount the caliper onto its bracket taking care not to damage the pads on the leading edge of the disk. Once the caliper is in place, secure it with the 7mm hex bolts.
Re-attach the brake line to the caliper, making sure the copper washers are in place. Once this is done, remove the molegrips.
Now go and do it all again on the other side of the car while I have a cuppa! LOL
Once all this is done you will need to bleed the brake system. I, being a total novice at this, used an EZIBLEED kit to do this and it was a piece of cake. If I can do it - you can do it!
Once done you should see something like this:

Note: The Omega doesnt use a sensor on the pad to monitor brake wear so you now cant either. To prevent a warning light on the dash, cut the two wires going to the sensor and join them together. Hey presto - no warning light!
Thats it - you're all done!

If I've missed anything out then just swear fluently while sitting under a car with no brakes. I did and it was great! LOL
Hope this helps guys.
Day