The recent AIC (available at
http://www.ais.org.uk/aes/pubs/aip/pdf/aic/4W378.PDF )
seems to define the content of the revalidation flight with an instructor.
I would suggest that you need to fly with an instructor until you are happy with your standard in performing all the operations listed in the AIC. Then have the 'revalidation' flight. Chances are you will need no more than two hours revision before you are ready for the revalidation flight. If you do expect it to take two flights, and you can afford to, you should slip an hours solo consolidation in between the two flights. This will build confidence, and will count towards the 6 hours P1 you need to revalidate by experience.
If the instructor is concerned about any aspect of your flying during the revalidation flight, there will be plenty of time for you to receive instruction to correct any deficiencies, if you have done a couple of hours revision prior to the flight.
On the other hand, you may find it cheaper to do a Proficiency Check with an examiner, - if you have done no flying in the last 12 months, you will have to do 6 hours solo in addition to the dual to get to test standard plus the test flight.
Best wishes for getting revalidated.