I have also contacted Turbo Dynamics. I have no prices yet (let you know when I do) but they said they are developing a 'wastegate only' rebuild/fix which will be ready in a couple of months.
Maybe te wastegate fix will eb £200 per unit but I would be surprised if they aer rebuilt exchange turbos for that as the lowest price I have seen is on eBay for £300 per unit from some company in Latvia.
Edit: Just spoke to TD and they are £395+VAT per unit but no exchange so need your turbos to rebuild. Also currently no wastgate fix.