I am relatively new to all this and am about to start an ad campaign, but I can't choose between Google or Yahoo.

I also was advised to hire an internet marketing agency rather than doing it myself through trial and error, not to mention the cost.

Because of my inexperience and low finances I would like to know if anyone has any advice about marketing agencies in the UK for running the campaign, maybe on a percentage basis rather than a set fee like a friend of mine in Australia uses.

Can anyone recommend an agency, or agencies in the UK who are good at what they do and won't bleed me for cash, which I haven't basically got.