Another thing to bear in mind is to steer well clear of any SEO agency that promises you "top position for x keywords" or something similar. Nearly all modern search engines used personalised searching, meaning that everyone sees websites in slightly different ranking positions depending on their personal search history. Absolute rankings are a thing of the past, any anyone claiming to deliver this kind of service is lying through their teeth. Modern SEO will use sustainable "white hat" techniques in order to bring useful traffic to your website, not use exploits and dodgy tricks, which will constantly get them slapped by search engine updates like Google's infamous Panda and Penguin algorithm changes.
Once you've found an Best SEO Agency In Essex that looks mostly all right you'll need to ask them some questions before you leap into their services. As well as the usual checks like existing client list and testimonials, it is a good idea to speak to someone directly on the phone or in person. Here are some good questions to ask them.
Who does their search marketing work? Is it handled by in-house staff? Do they do any outsourcing, especially to foreign companies? A lot of agencies outsource to cheap link builders in Asia who tend to produce very low quality work that can have a negative impact on websites in the medium to long term.