Thanks for the replies guys.
I couldn't get the cookie cutter to give me very good results - but shapemerge is not a function I have used often and as the project is quite time critical I don't have time to get to grips with it. The conform tool sounds like the perfect solution but I'm guessing this is in max..for the moment we're running VIZ.
However, booleans have provided a solution.. I extruded the field mesh to above the terrain and the cut -> refine the two. Edited the mesh to remove the top vertices from the extrude which left the inside of the extrude with the vertices from the terrain and then capped the holes. They will take a little jiggling to get right but is has given me the field shapes to work with on the terrain. It's probably not the best way of doing it but has worked so far. Thanks again for the replies gave me a new angle to think through the problem which I seemed to have managed to solve (sort of
)
Geoff