Hi Jo,
If you are using v2 its pretty straightforward, click on the course, then click on users under settings, then Enrolment methods then edit. If you are using 1.1 then I think you have to go into the code - if thats the case:
Course Welcome Message in 1.1 seems to be a language string: welcometocourse and welcometocoursetext in lang/moodle.php
regards,
George.