I got the above error solved, I had neglected to put the correct PayPal email address is the live site, after setting the APP to live on the store.
I have however discovered another error which comes about when the sellers commission works out to have 3 decimal places. This error is received:
This transaction cannot be completed at this time. Error message from PayPal: If the fractional amount is rounded for currency conversion, funds could be lost If this error continues please contact the site administrators.
Is there a way around this error other than changing the price of the item so that it doesn't have a commission working out to 3 decimal places?