Hi Phil!
I would recommend having a look at these articles:
http://kb.x-cart.com/display/XDD/Step+1+-+creating+simplest+module
http://kb.x-cart.com/display/XDD/Step+2+-+applying+design+changes
as they will help you to create a mod that will do the trick.
If there are any extra questions, feel free to ask.
Tony.