OP, firstly, why hadn't you paid when notified originally? Making a habit of delaying can, as here, create problems.
Secondly, submit a FoI request for copies of correspondence sent to you within the relevant time period. You could also simply ask for this. Ensure you can prove your identity.
Thirdly, 'which they say is now due on top of the council tax'. How have they 'said' this. If you conduct your enquiries by phone then, as this is the devil's weapon, more problems are likely to come your way. But if you have correspondence, then pl post here.