What you need is one of those metal poles they use for putting tape etc around things at roadside etc, councils use them a lot, straight "rod" about 1metre long with a loop at the top, (think about it and you'll see what I mean

) now cut the loop off, put a piece of rubber pipe [tight fit] over the top {this protects the inside of the flag pole} and sharpen the point up a bit, there you have the "anchor" hammer it in the ground and slide the flag pole over the top.

Ours has withstood some very high winds like that with no further anchoage required.