Thanks for the tip Ryan!
What it needed in the end was this:
$form['autocompletehack'] = array(
'#type' => 'markup',
'#value' => '<script type="text/javascript" src="/misc/autocomplete.js"></script>'
);Works a treat 
I'm going to have a fair few bits and pieces to contribute to Ubercart once I've completed this project - this Ajax search, a sophisticated stock control module allowing tracking of a product's quantity and cost price on a per-supplier-invoice basis (e.g. you get a new delivery in and the cost price has changed slightly - can't be handled at present), an option to remove "Add to basket buttons when a product is out of stock, and no doubt more by the time I get to the end! This is a great project and I'll gladly do my bit!
Regards,
- Paul
Joined: 03/11/2008