Formatted Fonts (Arial) Exporting to different sizes

Using jasperserver 5.2 CE.

All reports formatted using IReport 5.1 and using Arial exclusively (all 8 point) with the font installed as a jar.

When exporting using XLS or XLXS fonts that are set to 8 point arrive in Excel as 7 point BUT ONLY when the column is formatted (.ie a date or currency field). Exporting to PDF is fine.

Can anyone point me to what I need to do for a correction please?

Thanks

Mark