[evingar]

I've always found it perfectly adequate to use the mains earth, but it may depend on your installation. In the last house I lived in, there was a strap down to earth in any case, via a thick twisted copper conductor from the back of the fuse box down the outside wall of the house (over head lines, old installation - originally late 20s, early 30s).

The problem with your own earth is it is likely to be variable, depending on soil resistance / dampness (etc)