I would initially concentrate on the job in hand, later you can add a sites under stress register and add Thornborough, otherwise you will be diluting effort from the start (even thought it seems small).
The question about the wording, is that you can ask people on the petition if they want to become a member, which is great in terms of showing you are representative of a larger number of people, but not so good if all they give you is a postal address.
Take real care over the wording of the petition - what is the real issue? It is that you want Silbury fixed, how EH go about doing it is up to them. The thing to remember about petitions is they are a one off opportunity for signatures. If you have to go back and ask for people to re-sign because the situation has changed slightly but is still not satisfactory, you will get fewer responses.