You are left in an impossible decision if you are unable to afford legal action. It is a civil matter so the only recourse is through the courts - I think you know this.
In terms of things you could do at no cost are:
1) You could write something along the lines of - I note that your fence and shed are in contravention of the convenant on your land, placed upon it for the benefit of present and future owners of my land. Unless you remove the fence and shed within 4 weeks, I shall have no alternative than to consider legal action to enforce the convenant. (Whether you then take legal action can remain your decision - the letter doesn't say you will take legal action - might make them sweat a bit). Obviously if you can afford a solicitor to send the initial letter it might have more bearing and be more appropriately worded in legal speak.
2) After a month, if nothing happens, the second leverage point you have (and it may be worth nothing in their eyes) is one about disputes with neighbours being declared when they sell the property. So a second letter could say - I note no action in regard to my first letter. I am sure you are aware that disputes with neighbours must be declared to prospective sellers. I consider that ignoring the convenant to be such a dispute that you should declare. This may make your property unsaleable in future, or reduced in value. In light of this you may wish to consider your position regarding the fence and shed.
That's as good as it gets.