Received a letter today 07/07/2020 from a collection agency (MIL collections) for an outstanding debt of £215.00 which they claim was owed back in 2016.

I have never had an account with Shell Energy and presume that they are claiming in the capacity of the former company they took over from First Utilty?  I did have an account with first FU in 2016 however I have documentary evidence proving that I paid in full and final settlement.

My question is , 1. Are they still allowed to chase after 4 years and

2. Is it legal if they are now a different company ?

3. what should I do next if anything?  I have contacted MIL collections (Truro Cornwall) informing them of this to which they seemed very uninterested and tried to pass the buck.

Any comments or suggestions gratefully received.  Thank You