You can book an MOT test up to one month minus one day before your current MOT certificate expires. Booking within this window means your new MOT expiry date will remain the same as your existing certificate. For example, if your MOT expires on August 10, you can book the test from July 11 onward without changing the expiry date. It is possible to have an MOT test even earlier than this, but if you do so, your new MOT expiry date will be based on the date of this early test, potentially shortening the period of the MOT validity compared to your current expiry date. So, the earliest you can book your MOT while keeping your renewal date is about one month before the current MOT expires, but you can book it earlier if you don't mind adjusting the renewal date.