I'm just away to send me gear box to Stealth Racing to get my Autotech Wavetrac differential fitted.
This is not cheap but mine's a track/fast road car so hopefully it will be worth it.
Cheapest I've seen for this is £800 for the differential and fitting kit
Stealth Racing do the fitting for £399 + VAT. This might go up depending on what they need to do to the box when they split it