Oh dear, diz - you do have to realise that every NHS vacancy these days attracts loads of applicants, and sometimes it's quite small errors in applications which lead to them being weeded out and not interviewed. Your CV and covering letter should reflect what you can do, so I don't know what you mean by that in your letter. Practice managers are (generally) highly professional people these days, so I hope you haven't shot yourself in the foot by sending that email.
It could well be that you are overqualified, and they didn't think you'd stay - but murray is right, for reasons of information governance and confidentiality, it's rare that employees are registered patients in the surgeries where they work. If that's the case, they should have made it clear to you.