I did wonder if you'd managed to sort this problem or not.

One thing I never thought about on your last thread, is the shed earth good?
You could have all your grounds connected perfectly, but if the grounds aren't actually connected to anything, then they won't be doing much good. A quick and dirty way to test, is plug an RCD in, and try the test button. If the earth is bad, then it won't trip.
Even if it does test OK, and the shed doesn't have it's own ground rod, I'd still be inclined to add a ground rod.