Hello
I am not at West Sussex area, actually I am in Spain right now, but due to how the internet works, usually the physical presence is not that important any more with websites related works. Currently I work with some companies from China, London, United States and Argentina. The communication is done trough email or skype (writing or talking) and usually we get along just fine.
I can do anything you mentioned in the requirements (speed tweaking, layout, security checks, plugins, css modifying, onsite seo.
The only problem with this kind of ongoing jobs is that both sides have to be happy with the work. I prefer to work with those kind of people whom I can get along well, and I am pretty sure this is also a requirement from your side.
To be able to start working I would need a full backup of the current site, (and also you should make sure to have one before handing over the site management to somebody you don't know :) It's safer that way, from a backup you can always get the site back...). From that I usually build a "working server" on a hidden, password protected server, where we can do the dirty work, without bothering the live site. Only after you are happy with the modification, they end up on a live site.
Feel free to hit me up for a chat, ask questions if you have, I would be happy to answer them
Best regards
Eva