I don't know how you feel about advertising, but if you are experiencing high amounts of traffic you could easily monetise this site with some Google Adsense ads displayed in a few places on the site.
You could also easily set it up so that the google ads are only displayed to non-registered / non-logged in users.
Each time an ad is clicked, you make money.
Add in a few non intrusive CPA offers related to cars in general and you could easily make a LOT more than the £250 / month target you've set.
Ps. Hello everyone! i'm new here, getting a new 320d sometime mid december so i've been lurking for a while, i just thought i'd chime in with my thoughts, i'm an internet marketer and do it for a living.