Debt collector harassment. URGENT HELP!

I'm hoping someone here can help.

Here is the background: In September I used the Dartford Crossing a few times (these were desperate trips to Kent to try and save my then relationship - which failed) and I was late paying it. I had a letter through with a fine but not until October and before I had responded I was taken seriously ill. I was in hospital for a while and then in bed for several more weeks and mail was left unopened, so I undoubtedly missed letters telling me it had escalated.

In December I had someone turn up at my home apparently from Marstons, and they gained access to my conservatory as I'd forgotten to lock it though they couldn't access the rest of the house. I was staying with a friend to recouperate at the time but they called me, from inside my own home, telling me they would take my car if I didn't pay £425.50 for the escalated Dart Charge fee. I paid it, wiping out the last of my overdraft, kicking myself for not dealing with it sooner but figured it was a lesson learned, and assumed it was over.

Then in January, just 3 weeks ago I had two more 'officers' arrive at my door also asking for £425.50 for an unpaid Dart Charge. I told them I had paid it but they said it was for a different one, also a September date. By this point I had no clear memory of what dates I had crossed and I was genuinely scared of these men. I didn't let them in but fortunately I'd just started working again after my illness so happened to have enough money in my account to pay them, so I did. I asked, multiple times for assurances that there were no more outstanding charges and I was given them, verbally, by the only one of the two very large men who spoke. I never allowed them to enter my home and I paid by card on my doorstep.

So now, Marstons have received £851 from me for late paid dart charges. Again, I figured it was over, lesson learned. The end of last year is a dark part of my life I am just happy to move on from now.

However today I was phoned again by someone claiming to be from the Marston Group (a mobile no) who was apparently waiting outside my house to seize goods to cover fees of £600+ (I can't remember the exact amount). I was stressed out my brain - I was at the GP at the time being told my son may need to be admitted to hospital - so I asked for half an hour to get home and medicate my child. I asked what address he had - turns out it was the one I moved from last week so I told him that I had moved from there, but not my new address. He told me there were officers also on their way to my new house - indicating they knew the address. That was 5 hours ago and no-one has turned up...

I'm terrified. I have no money right now - I'm self employed and it's going to take many months to climb back from three months off sick. I have had no letters this year at all saying claims have been made. I had no idea this was coming. Again they are citing September dart charge dates but I thought I had already paid them. I can't believe I owe another £600+ in fees.

My Dad rang the CAB for me today after discovering me in a quivering mess earlier. He paid £10 to do a search with Registry Trust Limited which shows that there are and never have been any court orders made in my name at my old address (I was there for over three years until last week). We have also rung the police and I am now waiting for an officer to come round (classed as none urgent so may not be tonight) to take more details and offer reassurance. I'm a single mum of two with a very sick kid, on the poverty line, barely holding it together and at any point I'm expecting a knock on the door from a couple of thugs in black.

What can I do? I have paid £851 already and I need this to go away. Other than my overdraft I have never been in debt and I've certainly never had to deal with things like this. How do I stop this?