Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
Were it a straight bequest then there would be no tax due as distributions would me made after any Inheritance tax (IHT) was paid. Trusts are more complex and it depends upon the type of trust. the Gov UK web site gives a summary here:
Unless this is an employer benefit trust then the application of NI appears a tad odd and the executor should be challenged.
The executor should be paying you the benefits from the trust after deduction of tax and NI, the latter unlikely, and advising you of the calculation rather as an employer does with a wages slip. If there i much more shilly shallying on the part of the solicitor report their antics to the Law Society, their regulatory body. Be firm with them and demand your trust benefits or face the consequences.
I do hope that I have shown you a way forward in this matter.
Delighted to have been of assistance. As a starter for ten ask them for their statutory authority for such antics.
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You must ask and they must tell you; in the scenario of your question I think this most unlikely.
Thank you for your support.