This is called an overage provision. It is not uncommon if land or property is sold for a purpose other than development and is then subsequently developed.
Are you actually doing any development or simply selling the property as it is?
Is the buyer happy to buy with it as it is or is the buyer the one who is insisting on this?
I’m sorry to tell you that no buyer will buy the property without the consent. That is because of the wording of the provision which says that he can’t be registered without written consent Of them or their solicitor.
The buyer could buy it and the provision will go with the land, but they would not be able to register the property.
You are going to have to either find the person with the benefit of the provision or make an application to court to get the provision lifted in the absence of the beneficiary of the provision.
If you are making this latter application you will need to prove to the satisfaction of the court that you have made all reasonable attempts to trace the beneficiary of the provision which would include instructing Private investigators or tracing agents and putting a notice in the London Gazette and the local press.
Can I clarify anything for you?
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It says that you need the consent of Mr D and Mr A OR their conveyancer.
You have the consent of Mr D but not Mr A .
Because the solicitors mentioned in the land registry document is also your solicitor, there is a potential conflict of interest which is presumably why they are not signing the document.
What you might want to do is ask them whether they could give consent if you had the file transferred to another firm of solicitors and the rest of the file was dealt with by the new solicitors and this firm was then acting independently on behalf of Mr D and Mr A.
At the moment, your existing solicitors seem to be coming up with all the obstacles without making any suggestions as to how this can be resolved.
If Mr A has died, then the executors could deal with this.