If you're going to drive your car away from the test station with a VT30 failure, there are some regs. It is illegal to drive a car of MOT-testable age that does not have a current MOT test certificate on public roads, with the exception of driving it away to a place of repair, (which may include your home residence). From there you may be allowed to drive to a pre-booked place of repair, and to a pre-booked MOT test station. So a couple of grey areas there.
Free re-test periods are at the discretion of the testing station, any remaining days on the old MOT still stand regardless of duration.