Also had a reply from Quaife direct;
The QDF20R for the G40 is £645.00 plus VAT (if applicable), but there wouldn’t be any discount available for a single diff I’m afraid
We do offer discounts when buying in bulk, so if you can organise another group buy on the forums then we would be able to offer the following quantity discounts.
2-5 diffs: 5%
6-9 diffs: 10%
10-19 diffs: 20%
20-29 diffs: 25%
30+ diffs: 30%
These would need to be ordered through a single contact and delivered/collected all together in a single shipment.
It can, however, be a mix of any diff, so not necessarily all G40 diffs if there are others with different models wanting a diff.