Padding number with leading zeros in XSLT 1.0

We have a number in XML that can go up to 3 digits in a large XML file that has to be converted to fixed length text for loading into another system.

I need to pad this with leading zeros to a length of 15 in the output (which is fixed length text)

Examples:


 - 1 becomes   000000000000001
 - 11 becomes  000000000000011
 - 250 becomes 000000000000250

I tried this:


<xsl:value-of select="substring(concat('000000000000000', msg:BankAccount/msg:Counter), 12, 15)"/>

to get the 15 zeros at the beginning and take the substring but I must have made a mistake with the substring because in the results I get


0000000000000000000000009LLOYDS BANK PLC
00000000000000000000000010LLOYDS BANK PLC

I also tried format-number but I it returns NaN


<xsl:value-of select="format-number(msg:BankAccount/msg:Counter, '000000000000000')"/>

returns 'NaN'

so what have I done wrong and what is the best way to do this?