No is the short answer. You need to get rid of the grass before you start growing - needs to be removed, the soil dug over and humus rich materials such as soil conditioning compost from the garden centre, garden compost, composted animal manures (although root crops don't appreciate manured ground), etc.
If you're growing in pots in the polytunnel, the grass will continue to grow in between pots, so still needs to be removed, or have slabs laid over the top.

thanks bamboo eamonn

And don't dig the grass clippings in you need to compost them first.

If the digging will be too much and you have the patience have a look at http://www.charlesdowding.co.uk/node/67
there's always more than one way to skin a rabbit, lol.

You'll still need to remove the grass though or it will grow up through whatever you put on top. Consider creating raised beds, lift the turf, turn it over and the add you organic matter on top - will need to be pretty deep to grow potatoes but they are best grown out of a polytunnel.

<p> thanks, moon growe and wagger

Hello! I would initially weed kill all the grass, then can you afford a man with a rotorvator for a day? It would save alot of hassle and be done with quickly. Then you can dig in all your manure etc, or maybe ask the rotorvator man to include it in his quote for the day? Most of these people have a day rate, so use it fully!