Unexpected resurrected debt

To cut a long involved painful story short between April 2014 and August 2016 my life fell apart. I ended up failing narrowly to kill myself and went to hospital for a prolonged stay. I fell into a lot of debt, culminating with NatWest repossessing my home. I also accrued other debts, credit cards, and importantly here utility debts, plus HMRC, and some others.

When my house was sold I was told all my debts were cleared. There was oddly enough no money left over, the fees came to exactly that amount, although I never got a letter detailing that.

I had owed British Gas around £2500, they had pursued me for it, but after the house was sold I never heard from them again. I got no threatening letters, no quarterly reminders of my debt, no half yearly reminders of my debt, or an annual statement of my debt. They know where I live, I'm on the electoral roll, they have my phone numbers. I didn't hear from them for most of 2016, and all of 2017, 2018 and 2019. Nothing. Then today I get a letter saying they've handed my debt to a third party, who now want a few quid short of £3000, payable now!

While this has been going on, me thinking I'm debt free, last year I did a last gasp, "I'm sure there's nothing left to claim, so let Gladstone Brookes have a try, I might get £50 back". Except they hit the jackpot. £4250 near enough. Except NatWest have tried every law, loophole, etc in the book to keep it.

Is it coincidence this "debt" suddenly resurrects itself? Can they do this? I would expect at the very least to have received at least one statement and the odd threatening letter in over three years?

After my suicide bid I'm considered fragile, a vulnerable adult. I've rebuilt my life a little, I live very quietly, unable to work due to illness, I'm going blind, I was going to spend the PPI money on adaptations and gadgets to make life easier for myself. That looks a dim distant prospect now. What was left was going to be my safety net against unexpected costs, or the nightmare of my looming PIP renewal, keep me going while they sort it out and I win in court, again.

So my question is this, can they resurrect a debt that was to all intents and purposes covered by the repossession, that they've not told me about being still owed in just under four years?

I know NatWest want me dead and buried, they sent debt collectors to the mental health unit to continue their assault on me, they had to be chased off by the Police. They weren't impressed. Now British Gas drop the 3000lb weight on me out of the blue? I'll be honest, this is exactly the sort of thing that undermines my recovery. I was coping with life, getting through each month without debt, existing on ESA & PIP, it's a finely balanced thing, I cannot cope with big changes.