Illegitimate adult child claim on inheritance

[RelativePitch]

My dad had an affair in his first marriage about 60 years ago. A child was born and the mother was paid off by my grandfather (around £250k in today's money). My father had nothing to do with this child and never met him.
My father died last year with a will whereby everything goes to my mother, but his properties go into trust for his 5 children.
My mother's solicitor rang to say that an anonymous woman rang up to say that she was going to make a claim on the estate on behalf of her son. No detail other than that. The Solicitor wouldn't commit to how worried we should be.
I understand that minors should be able to claim against an estate, but a 60 year old man?
Of course it could be another child we don't know about, but as my dad had a vasectomy in 1984, the youngest a child could be is 40.
My dad makes Boris Johnson look like the paragon of virtue!
How worried should we be?

But even with the trust for some of his estate this child of his has no legal claim if he hasn’t been named in the will. 🤷‍♀️. Parents can disinherit their kids in England.

Not saying what you said at all. Bizarre, I say keep things simple. Yes, people have choices. It’s sensible if those choices protect a spouse, if they are the one and only ever spouse. It can be more complicated, but it’s best if it’s not too complicated.

@JanewaysBun my mum isn't on any of the property deeds. She has her properties and he had his, no joint bank accounts, everything separate. She is not allowed to sell his properties. She will have the rental income. It's an interest in possession trust.