Hi Old Bird,
Have you seen those 'metpost' things that you can get in B&Q, basically a metal socket and large spike that you bang into the ground. The socket varies in size but fits most square sized sawn timber (such as 47mm square). I'd make the table legs using 47 or 75mm square timber and use a couple of these metposts to anchor it to the ground, probably easier than burying the frames legs and timber buried in damp ground rots away eventually.
Good luck!
Jon