Above all else, keep in contact with them and do not ignore them.
1. Ask them to prove the debt is yours
2. send them a letter explaining your situation and enclose your monthly budget.
3. Provide your contact preferences whether that be by phone or letter at specified
4. ask npower and moorcroft for an interest freeze
5. ask them to accept token payments
6. if you cannot get the money together ask them to accept a lower payment amount.
7. contact a debt charity to consider your options and perhaps look into insolvency options as a last resort.
Full guide here with all the above information in more detail: