I wouldn't recommend black polythene - whenever it rains, your gravel will be swimming in water. Use a proper geotextile membrane instead. Should prevent the grass growing through. Obviously, you know it would be better to remove the grass first...

If you're prepared to use weed killer. Weed killer the grass. Leave a few days to take effect and then cover with a semi permeable membrane as suggested by Bamboo.

I've used that technique a number of times and never seen regrowth of the grass . It saves alot of work

I have seen black polythene used which was spiked to allow drainage but it had a 4 inch layer of gravel over it so any trapped water never became visible.

the other thing with polythene is the gravel will slip to the lower spots unless underneath is realy level . even with a membrane wich is a much better idea you realy need to dig down some and have some sort of edging so you dont lose said gravel . i agree with bamboo sometimes there are know short cuts .if i was going to do that id use something like broken slate and put some edging round wear your doing it not that io would .

thanks for the info guys

your more than welcome .

you can put a membrane down on top of the lawn, and if you put a small edging around the lawn, you can then fill it up with decorative gravel. the grass will eventually die away.
if you use decorative aggregates you can also put a few standing stones or rocks and you can also get small easy to install water features, this can really transform a plain old lawn and give a fantastic visual effect

have a look at some ideas in the gallery at
www.decorativeaggregates.com
and they will deliver to your home