Your LHR - DEL allowance will be your allowance for your entire itinerary if this is on a single PNR. 3x23 kgs.
That's because 9W baggage allowance for LHR-DEL sector is fixed depending on your point of origination. So your LHR-DEL-KTM & DEL-LHR will be @ 3x23kgs. By IATA rules and 9W's internal allowances they will allow you 3x23 on the KTM-DEL flight as well.