format number with changeable currency symbol

Hello
I would like to format a number so that it can be shown with any type of
currency symbol in front of it.  Currently I am using eg. <xsl:value-of
select="format-number(12345, '?###,###,##0.00')"/> but this only formats the
number with a ?.

Also I have tried putting a currency symbol element in the xml and including
that in the xsl to be inserted before the number.  The trouble with this
method is that if the figure is a negative number the minus sign is inserted
after the currency symbol which is no good.  Is there a way that the
format-number function can be used with a currency symbol element from the
xml so that the currency symbol can be changed in the xml?  Or is there any
other way to solve this?

Thanks for your help.
James