Thanks for the very quick reply, fxDreema.
I was using downloaded tick data from Dukascopy, and transforming it into MT4 history files using the excellent Tick Data Suite.
After I'd done some more research on this problem I decided re-install everything, and also give another broker a go. The first broker confusingly called its pairs GBPJPYSB (spread bet), whereas the second used the normal GBPJPY (even for spread bet). The tests seemed to work fine with the first broker (99%, etc) apart from the "Tester: exchange rate cannot be calculated" error at the start.
The key, therefore is the name of the pair. If the name of the history files don't exactly match the pair name the broker gives them (in my case GBPJPYSB instead of GBPJPY), then this error occurs.
Birt (of Birt's eareveiw.net - home of Tick Data Suite) said: "Just for future reference, FXT files created while connected to a broker that uses a suffix for the currency pair names will not work properly on brokers that do not use a suffix and vice-versa. In case you have to perform such a switch, you either have to recreate the FXT using the new broker or you have to hexedit the FXT and remove the suffix (or add it)."
Hopefully this information will assist someone else in the future who stumbles across this problem, and searches for an answer.
Anyway, all's well now.
Thanks again.
