Unfortunately, if the neighbours passionately want to prevent development of your land, they can dig in their heels and refuse to discharge or modify the covenant. The fact they want a lot of money suggests it’s not about the development, it’s about the money.
If you can’t reach agreement, remind them that no build equals no money for them. If all else fails you can apply to the Lands Tribunal for discharge or modification of the covenant. You will need to take specialist legal advice before heading off down this route.
Where a covenant impedes some reasonable use – such as building houses with planning permission – and it’s not in the public interest to prevent the houses being built, then the Tribunal can modify or discharge the covenant. They will also determine a suitable level of compensation for the neighbours.