want to book an early car practical test?

The web-site is www.direct.gov.uk i think the option is on the right hand side of the screen under 'Do It Online', 'Book Your theory/practical test' I know you already have a date but if you click on this you get an option which says something like manage your booking. This allows you to search the tests available at the test centre you want, and then if there is an earlier one you can switch. They should have sent you a letter with all the codes you need, otherwise i think it's your driving licence number and your theory test number you need to enter.

A good time to check is lunch time, well when i was trying to switch it was! You get three goes to swap as well so you've got nothing to loose.

Good luck.

Yes there is an online service but you need to ensure your car is available then or you could end up with a test date and nothing to do it in

You could try a different test centre ,some are a lot less busy than others.Good luck with your test.