House value for probate with Deed of Trust

I am no expert but it seems to me that the 1989 restriction could have been a Form N restriction.

I assume that there was no mortgage - your mother had cash and your brother had cash and they each put up half the purchase price of a property.

There was no formal documentation of any form of loan nor was your brother registered as a proprietor of the property.

His protection was that your mother could not sell without his consent - if he had given such consent and the property had been sold, it is unclear to what proportion of the sale proceeds he would actually have been entitled.

Just the repayment of the money he had put down? Fine, the repayment of a loan. Half the proceeds of sale? On what basis - a form of rolled up interest? Income tax may have been due. Otherwise, there was a capital gain and CGT may have been due. Or perhaps, with no form (apparently) of written agreement, nothing at all?

It is possible that the above may also have been relevant at the time that the DoT was drawn up.

With regard to the DoT,

https://www.rocketlawyer.com/gb/en/quick-guides/declaration-of-trust

may be relevant.

It would seem likely that your brother will need to consider his CGT position.

It could be a good idea to obtain professional advice - it may be that HMRC would give some guidance as to what to put on the form

https://www.gov.uk/government/organisations/hm-revenue-customs/contact/probate-and-inheritance-tax-enquiries but equally you might be told to obtain professional advice.