Existing soundproofing behind TD5 dashboard

So I want my TD5 to smell less like an old barn so I removed the dashboard and realised this would be a good time to think about reducing the noise as well. Having everything off I've found there's a load of black material behind the upper part above the footwells. It's looks like 4mm vinyl and it's being held on with a bunch of sort of protruding rivets. It's all falling to bits and I'm going to get rid of it but I haven't spotted it on any of the parts diagrams. It looks like it's pretty old so I'm assuming it's an LR thing, but what can I do about the rivets? Can I just drill them out and bung them up? I don't want anything important to fall out of the engine bay just because I got a bit keen.