Walker Love set up payment plan they took payment once and not since (I didn't notice)

dekaspace1

I have a debt of £300 (almost all fees btw as the rest was paid off despite disputed)  Back in January when I found out I set up a payment plan of £15 per month and after 1st payment was taken in February and to be fair I haven't checked since but I got a letter yesterday about an arrestment of earnings for £120 but it listed this year as the bill, as I work and the wait in queue is about 30 minutes a time I barely got a chance to contact yesterday and they said they couldn't confirm what the amount was for so I contacted my local council who said the £120 was as I had 2 properties last year (I had a overlap of a month when I moved house which I made them aware of at time and they said thats ok as I didn't live in both properties at same time and sent me out a form to fill in and put arrestment on hold.

Anyway when I got home I had a letter for the other account number (previous address) from Walker Love adding £69 in fees which is why I found out they haven't taken payment.

What are my rights? They had correct details as they took payment once, they never phoned, emailed, or sent letters to me to tell me the payments weren't going to them and this is first I heard it hasn't been paid. I appreciate I should of kept an eye on it but I have disabilities that make me confused easily and genuine mental health issues.

Anything I can do?