If he purchased a month or so before the date on the register you have then chances are that the register was downloaded before the registration of the purchase in to his name occured. Tell him he should download a more recent version and produce this to you.
He can do this here for a small fee:-
It will probably have him registered section B as the registered propreitor of the title. If so, no problem in selling. If not then he needs to contact the conveyancer who acted for him and providing he had a right to be registered as the propreitor of the title he will have a case for negligence agianst his solicitor and they will do everything they can to register it in his name.
There should be a clause in the Lease requiring the purchaser of the interest to send you a Notice of Assignment upon sale of the property so that you can update your records and know who to contact with regard to the collection of monies owing from leaseholders to you under the terms of the lease (eg. service charge, ground rent, insurance commonly. If this has not been done then it is a breach but if you have not suffered any loss as a result it is a fairly immaterial breach.
If the lease does not contain provision regarding your consent to the sale (and it almost certainly won't) then they can sell as and when they like.
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