Good and Interesting thing that you are giving ads for as little as £20, but my confusion is that whether this charges are for per ANNUM or per 3 MONTHS.
So please be sure that what you will charge and for how much time.
Second question is that your website even don't have a page rank or not so much presence in the internet world.
You even have not provided any traffic & visitor information about your site and there is less or no information is available.
I hope you will provide all these information so that Advertisers can decide before making and advertisement decision on your website.