Without knowing the precise wording of the covenant it is impossible to be 100% certain about the outcome. However, in many cases clauses such as you describe are what is called restrictive covenants, & are normally made in favour of someone who retains an interest in the locality - such as to protect the view from the vendor's neighbouring land or property. Such covenants can be overturned if the person in whose favour they were made is no longer in a position to benefit from them. Often, potential liability under the covenant can be insured for a small premium.
Your case seems to be different, as your granddad presumably moved away from the immediate neighbourhood. If so, his solicitor should have worded the covenant in such a way as to protect his interests so far as possible. The usual way of trying to ensure the vendor retains an interest in any development value is to spell this out in the covenant, but this is normally done for a limited period of years.
As yours was placed 40 years ago, it seems much more likely no time limit was set. It so happens that the purchasers have traced your granddad (perhaps they were advised they must attempt to do so), so I would be surprised if they could get the covenant removed without his agreement. If he had not been traceable (or had died, & they couldn't trace whoever inherited), I suspect they could have gone ahead without paying anything on the basis of indemnity insurance.
Legal advice, based on the exact wording of the clause, is vital. I would think that an absolute minimum of 25% of the land value should be obtained - possibly quite a bit more.