Unfortunately the area of ground I was standing on appeared to be the only area of ground! The holes problem that I have read about (but thus far not suffered) appears to be reversed, and all of the ground is missing bar the area where the player starts, and a couple of apparently random spots nearby. Everywhere else the ground is transparent (or appears to be X distance below me), while plants etc (only the default grasses) appear to be floating at my feet.
What did I do wrong and how do I fix it?
Cheers
MVK