This is returning the lowest price within each column and I want to return
  the lowest price over all columns (within the range). I apologise if my
  exaplanation was poor.


just change price[position()< 5] to what you want, which is
something like
 ../column/price[position()< 5]
if you mean the range applies to prices, or
 ../column[position()< 5]/price
if you mean the range applies to columns.

also of course I have some spurious brackets in
>    <xsl:if test="[$lowest= number(current())])">
                   ^                          ^^

David



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