"Surely this is negligence on the side of the solicitor? They shouldn't have released the funds if there were still matters outstanding?"
Yes, you are entirely correct.
"The solicitors were the executors"
This is the important part of your post.
If the executor distributes the estate before fully settling the IHT liability with HMRC then they assume joint and several personal liability for the remaining balance.
Once assets leave the estate, HMRC's primary legal recourse is against the proving executor, not the beneficiary. HMRC can go so far as to pursue the executor’s personal assets, bank accounts, or force the sale of their home to satisfy the debt. If the executor is a firm of solicitors then they will seek to enforce against the assets of the firm.
There was a case in 2018 that illustrated this, Harris v Revenue and Customs (Inheritance Tax: Other) [2018] UKFTT 204 (TC)
(otherwise Glyne T Harris as personal representative of Helena Norma McDonald (deceased) v HMRC [2018] TC06448 )
https://www.bailii.org/uk/cases/UKFTT/TC/2018/TC06448.html
In that case, Mr Harris was the executor for Helena McDonald's estate. He distributed a substantial amount (possibly all) of the estate to the beneficiary.
The beneficiary said "Trust me, bro, I'll pay any IHT due". The beneficiary then decamped to Barbados and refused to pay the IHT. The executor was held to be personally responsible for paying all of the IHT due, which was about £340,000.
The executor in that case was just an ordinary person who didn't take legal advice and was held to be liable. That a firm of solicitors has distributed the estate before ensuring that any IHT due has been paid really does not look good for them at all.
I would strongly suggest that you search for "Contentious probate solicitor [name of your town]". Then speak to them about what has happened and that the executors are coming after you personally.
You really do need legal advice that is specific to your particular circumstances.