I'm sorry! Just after posting my query I found the following:
http://www.webassist.com/forums/posts.php?id=28540 (Populating text fields from dropdown)
Must look harder next time, but using the example in there that you provided, I've just had a quick look and got it working with my data so I'm going to have a look tomorrow and tidy up my code and page and hopefully fingers crossed I've got it all working.
Always nice to have a satisfying end to a day :)
Sorry for the wasted post !