[lacewing]

I have just purchased a maiden whip apple tree and would like to train it as a step over. Could someone please advise how deep I should plant, should the soil be level with the graft?  There is about 18" between root and graft.

[NickoV]

You must keep the graft away from the soil. If the soil level goes over or too near the graft the root stock will start growing at the expense of the top bit(the bit you want to grow).
Lots of info on the net about how to plant an apple tree.