Contact npower Corporate

My family has just moved into a rented accommandation which had a pre paid gas card for NPower. Within the 1st 5 days we ended up paying £49 on the card.We finally found out why so much. out on £20 credit we were receiving £3 credit and £17 was paying someone else's debit. After 7 phone calls averaging 40 minutes a time and a lot of explaining we finally got some credit back.We tried to get the landlord to change this meter and they finally agreed to send a engineer,but as I put the account in my name he was cancelled.so when he eventually came out having taken time to change my schedule for that day he didn't turn up until 6 pm at night changed the meter then went. By now we decided to change power companies.We moved in the property on the 27th Nov,the pre paid card changed on 6th Dec and the new company{Scottish Power) took over on the 6th January 2018, NPower gave us £30 credit as goodwill gesture.also all the credit left on the pre paid card. Now they are trying to bill us for another £58. NPower are trying to say for 39 days gas it has cost a total of £137. Which is just not fair,we have been in touch yet again with NPower another 4 calls.and now the complaints dept, has been in touch with senior executive and has agreed this is the correct amount, I need help has it is a 3 bedroomed terrace house we not employed has my husband had a heart attack recently.and the money is very tight.how can i exploit NPower for this .