Is this the outward leg of a Burton-Manchester-Burton journey or the return leg of a Manchester-Burton-Manchester journey? Either way, there are no restrictions on breaking your journey on the available standard-class anytime return and offpeak return tickets. So this seems fine so long as you plan to travel from Belper to Manchester via Chesterfield. If you wanted to go Belper-Derby-Uttoxeter-Stoke on Trent-Stockport-Manchester, you would need to buy a Belper-Derby single and then use your Burton-Manchester ticket from Derby onwards. If you wanted to go Belper-Derby-Tamworth-Stafford-Manchester you would need a Belper-Burton single. This answer could well be different if you are using "Advance" (i.e. booked-train only) tickets.