I know for a fact that if they tried that on me I'd make them jump through every legal hoop possible! They made the mistake, not your Mother! Screw it, it wasn't even a mistake that involved her, it was something that occurred against your late step-Dad!
Surely there's some legal recourse she can take. 19K seems a bit rich considering the error wasn't even yours/hers!
It's a tax issue and when it comes to the ability to claim back money Inland Revenue is about as high up the chain as it comes.
So firstly - there is no telling them to "Screw It".
Secondly they are the law with regards tax.
His mother is not being asked to pay the money, it is coming from his late father's estate - this is entirely different.
If his father had £19k+ in money and assets which were passed to his mother then this is where the money is coming from - not his mother.
If his father didn't have that much then they can only claim back from the value of his estate.
When somebody dies all of their money and assets make up "an estate".
Then all of that person's creditors come forward and make a claim.
If the estate is big enough to cover all the debts then fine - all debts are paid and anything left over is given out as per the deceased's will or to the next of kin.
If the estate isn't big enough to cover all the debts then you either need to negotiate with his creditors or employ a solicitor to do so for you.
Once the estate is all used up the next of kin cannot be asked to pay any of the deceased's debts.
Anyway, the only thing I question here is the date.
They cannot claim back tax from that long ago.
Basically if you think they have messed up you have up to 5 years to claim.
If they think they messed up they have up to 7 years.
So if this is back from 1983 they cannot claim it all back.
Basically they can only claim back pay for 7 years prior to when you received notification.
So even right now they can only charge his estate back-tax to 2000.