Hi June - the restriction is known as a form A/joint ownership restriction and it is often applied for where there is a trust involved.
The subject is covered in a number of threads on the forum with a few 'stuck' to the top of the 'Ownership' section -
Ownership · HM Land Registry
If your MIL is now the sole legal and beneficial owner then an application in form RX3 with ST5 is required to cancel it - see section 8 PG 6 for guidance on the requirements and what form ST5 should include by way of facts
Devolution on the death of a registered proprietor (PG6) - GOV.UK - The letter from us to her dated 3rd June explains what's required for example
In answer to your specific Qs/concerns
No re JW writing to us to advise the trust is no longer on place. They might have advised that an application in form RX3/ST5 could/should be submitted but writing to us would not achieved not be an issue - for example she could appoint someone to act with her as a co-trustee on the sale and together they then transfer the legal ownership to the buyer and therefore comply with the restriction
And if she did pass away and there were two executors the restriction would not for example prevent them from selling and/or if they transferred the property to a beneficiary then they could apply to cancel the restriction instead
From a registration perspective I don't think she need have any concerns and would encourage you to try and allay those fears as appropriate
If you/she remains uncertain of what to do and how then I would recommend seeking legal advice/assistance as appropriate
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