When booking your tests, make sure you do it through the official pages from direct.gov.
Below are direct access links to the official booking service.
If you use a booking service it is likely to cost considerably more than the DSA booking.
The Official price for the Theory test is £31 so if asked to pay more than this your in the wrong place.
The Driving test is £62 so again if asked to pay more your in the wrong place.
Theory test booking.
Driving test booking.