If you are going to use a membrane you should ensure that it is porous otherwise you could get a build-up of water under your patio. Ask for a heavy duty weed-suppressing membrane at the builder's merchants or garden centre.
Most weeds will not come up through your new patio - Japanese knotweed is an exception. If you do get a weed problem it is likely to be from annual weeds that seed themselves in the pointing gaps. You can stop this for a whole season by watering in March with Pathclear.