Another reply from Bitstamp, they have confirmed that they are using the prices at http://www.raiffeisen.si/ceniki_in_tecaji/menjalni_tecaji/1 to figure out the conversion.

So for us brits, it's a nice bank to use.

Raiffeisen shows 10000 GBP = 17,086.7286 USD
Unicredit shows 10000 GBP = 16,969.43 USD

Just got to confirm that it's accurate information .