Hi Alan
Earlier versions did have issues with foreign currency on account/prepayments. If you run the Purchase Ledger report with function Rate Differences, does it show you any differences? Also, what does the Prepayment History P/L report show in base currency and currency?
Solving these currency issues can be quite hard. Sometimes upgrading to the latest version will fix some of the issues but maybe not all of them. It's probably best not to try to fix them in 6.3 because you might find your fix doubles up when upgrading.
Regards
Paul