Thank you for your response, but I can say categorically it is not a cache/web browser issue (am a web developer in the other half of my working day).

The 1:1 exchange rate is being applied at the time the receipt is matched on the banking screen. I have just repeated the action, and never visited the receive payment screen to check, but can see that a rate of 1:1 Euro->GBP has been applied to the receipt as this is shown on the realised gains and losses report.

It seems to me that their is a bug that the correct exchange rate is not applied at the time of matching a bank receipt on the banking screen to an outstanding invoice.