I have (douglas?) fir tree. The tree is about 50' high and the bottom 10' or so has been cut away (4 or 5 years ago)

The area under the tree gets pleanty of light, but the grass groes only sparsely. I've had it PH trested and its very nearly 7.

What should I be doing to keep the grass groing under there?

You will have to dump a lot of water on the grass to keep it growing under the fir, better to find some low moisture loving plants.

Thanks for your reply. I don't want to have to water it all the time. We are very nearly in drought conditions and the restirctions are inevitable.

I don't really want another garden area to maintain either. I'll go to my local garden center and ask for some medium light, slow growing, non thirsty ground cover.

It is a shame. It is a lovely spot with 25'diameter of cover from the fir tree. Perhaps I'll put play ground under there for the kids.

Thanks again!

You could get some low growing groundcovers that will stand up to some traffic, like creeping thyme. Stay away from english ivy, it's very invasive and will try to grow up the tree.
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I've had pleanty of negative experience with Englosh Ivy, thanks for the warning and the tip.