They can't embed any sort of script unless you give them access rights to do so. They would normally supply you with a code snippet which you would manually include on any page(s) the ad is to show on.
Since you are webmaster, you can remove the offending code in seconds if the content of the ad gets out of hand at all.
What you should be very careful about is any agreement of term and/or usage. I'd ask for a sample agreement document which should give you an idea of what they will demand for their year and £100 fee. You can't just go removing ads once you have agreed for them to be present on your site and accepted payment for it/them. Doing so could cost you deraly, depending on how their agreement is worded.
My rule of thumb: if in doubt do nowt.
My other rule of thumb: don't include ANYTHING on your site unless you really want it on there.
Ad scripts, by their very nature, often pull content from the parent site and so the code snippet may display something different every time a visitor clicks on your page. It could be a banner, text or whatever, but I think to even consider this for a moment you need a lot of very details information.
A year is a long time in many respects, and is a very long time to have to continue using ads that you have obligated yourself to.