Interesting then. So the land clearly
has been used for a use other than agriculture.
Should be possible to find out if any enforcement action was taken on this, in connection with the covenant. However as the covenant was enacted more than 40 years ago, and
if no action was taken on the last apparent use of the land, then this concern may be of less relevance than the potential pollution.
It all really depends on how much you want this particular piece of land. To me, land on your doorstep is worth at least twice as much as that 10 miles away and only you can decide how much it is worth to you personally. 30 years ago I paid building land prices for a quarter of an acre of land that gave me a rear entrance, with road access, to my back garden. I have never regretted it and that rear access for vehicles to my field is invaluable.
Possible contamination is not necessarily insurmountable. Look up ways of mitigating it. Again it depends how far you want to go. I assume the land is for sale at an attractive price as it doesn't sound the most visually appealing of plots.
As for the boundary - I would ask for it to be clearly marked out on the ground, and ensure the neighbours agree with it so there will be no subsequent arguments, before you buy.
It might be worth having a word with a solicitor about your worries to get things in perspective.