We are planning on just a civil ceremony!! I will have to look into residing in the cachment area of my registar, as that looks like the best option at the minute!! (Do you know what the paperwork is like for this option... is there any??)
As Gary said, a marriage visitors visa is required! While officially you have to reside in your registrars cachement for 5 days, there is no formal paperwork to "prove" this, usually the registrar takes your word for this. I simply showed up at the registrars office and made my appointment in 5 days time and the "proved" I was in the area. There is paperwork to give notice, but the registrar fills this out. Once you have given notice the dates can be somewhat flexible (they are hard period for Civil Partnerships for some reason) and you will choose a venue (registrars office is cheapest) and then come back for the marriage ceremony on your appointed date.
Plan for 20 days, call the registrar's office ahead of where you are going to registrar and talk through it. Most registrars are perfectly reasonable and friendly people, unlike some of the UK Borders folks. Be sure to inform that one person is a UK citizen and one is coming on a MVV.
Nothing to panic about, just obtuse, expensive and lengthy, where getting married in the States in most cases is perfectly easy, short and cheap.