Thanks Fred(!) for the fix...
I'll save the group a jump by echoing the change from Fred's page:
Bug fix with negative frequency offset value. Using a negative offset value in the LO calculation will not add / subtract anything, in the previous firmware version.
The bug is fixed, and the code still fit in a ATtiny45 chip (2 bytes free) also the ATtiny85 version is available.
A other small change is made to the band crossover point values. The previous version did store the si570 running frequency, the new version will store the set frequency. In the default case of offset=0 and multiply=1 there is no change. And also the command 0x3A will return the set frequency and not the Si570 running frequency.
The change is completely handled by the configuration tool ExtIO_Si570 version 0.9, use that or newer versions for this firmware version.