It is unusual for a covenant not to come up during the purchase. If it is a recognised covenant it will be on the deeds and if you were not advised of it at the time of purchase I would argue you that you are not bound by it later. Would be a very interesting court case if somebody tried to hold you to something you had not been aware of. There are several covenants on the house I am selling. I have had to list them on the Seller's Information questionnaire and they are also on my deeds.