I think you ought to see a solicitor for this sort of stuff in future, while the template may be fine you're not necessarily going to know for sure what is reasonable in terms of charges, interest etc..
I mean maybe all your invoices can be for double what you really want to charge but with a 50% discount if paid within 30 days - I dunno I'm not a solicitor but presumably there will be a range of ways in which you can introduce a decent enough penalty for late payment but also be confident that it can be enforced.