You can try a new foam seal. (Remove the sunroof, scrape off the old one, refit the new one)
But the chances are it will be the main rubber seal in between the glass panel and the main frame.
On an old T reg car, i'd personally silicone it all shut. Make sure it's bone dry, open the sunroof and squirt silicon all around the seal. Close the sunroof. Then fill the channel that runs around the glass. If you take your time you can actually do a really nice job and nobody will notice unless they get up close.