I've just bought a Autotech Wavetrac differential,
http://www.autotech.com/prod_drive_wavetracDiffs.htm
this is a ATB but with a bit of a difference.
I got mine for a good price and can point you in the direction of a few suppliers.
With this you'll need an 02M differential fitting kit
http://www.seatcupra.net/forums/showthread.php?t=241551
As for the gear box work there seems to be one main VAG guy who is on everyones lips when it comes to GB's and that's Vince at Stealth Racing.
http://www.stealthracing.co.uk/
From these guys your £399+ VAT for a gear box recondition and differential fitting. This might go up if there are any issues in the gear box.
The selector fork is a weak point in these boxes, ask them to drill out the rivet and replace this with a bolt/nut, cheers to Badger 5 Bill for this one
.
There are a few other companies that will give you good quotes for differentials, try
BackDraft Motorsport
INA Engineering
JBS (as above)
I have bought another gear box so can still use my car in the mean time. I'm just away to send my box down to Stealth in the next few weeks.
Let me know if you need any other information.