Electricity Company threatening toreturn payments

[seduraed]

I owe myEnergy upplyer Npower arrears for electricity. I am starting to pay the regular usage by DD but want to pay the arrears on an ad hoc basis as my imcome from lettings is irregular. The adviso said that unless I commit to a specific method approved by them they will simply return the payments and not credit them to the account. Is this lawful? what happens if I continue to pay and they keep returning payments?

[des8]

That is considered an unfair business practice by the FCA
https://www.handbook.fca.org.uk/handbook/CONC/7/12.html
Conc 7 12 2 (2)