I must have put my question wrongly about changing the default boot from XP to Ubuntu because I had a plethora of helping answers with very complicated methods to do it via Grub & Boot.ini.

This simple way in XP does it:

Control Panel>system properties>advanced>startup & recovery>settings>default operating system>click down arrow to show Ubuntu, click it & OK.

Next start up shows first Ubuntu above XP. .......nick