No way forward

I had an alarm system and armed response with adt fidelity and I wanted to cancel cos I found a better solution. I called them and also sent email on 27th of January 2023 and placed my 1 month notice of cancellation. The last payment was due 28th of February which I made. Adt fidelity acknowledged receipt of my email and request to cancel our agreement as it was over 3 years. Now I keep getting letters from them saying I owe them money for March till may. When I explained my situation and all the proof of emails I have the consultant said no problem will sort out. But still every month they are sending me bills and now even a very threatening letter from lawyer's. This is ridiculous. I owe them nothing but they want to take advantage when it's there own fault not updating their system. I am now pursuing legal action against them.