I knew to watch out for these but spotted my husband on one, although I suspect he'd have twigged when asked for extra money at the end of the form. The one you want is on www.gov.uk. beware of uk-passport.net which has an official looking logo and if you google passport application that comes up first and only tells you half way down the page that you have to fork out an extra £40 for their service (and their online form is inferior to the official one, less helpful). They also charge over £1.50 a min for their phone line compared to the offical websites 10p or so/ minute.
I got caught by the driving licence one yesterday whilst trying to change address. Managed to cancel it but am unhappy with the amount of data I gave them. They look just like the official sites. Check for the .gov.uk ending to the web address.