Probably nothing you can do. I, for one, will be surprised if this convenant is actually between you - I suspect the 'convenant' is a restriction written into the land title of each of the properties say says you shall do this and that. In which case it is likely to be a requirement that can only be enforced by the seller of your neighbour's property to him.
Have you asked him nicely about whether he realises the wording is there?
In terms of laws, this will be contained in one of the numerous Law of Property Acts.