Thanks to Foxster.
The law says that a plate has to be visible from both sides at an angle of 45? from the centre of the plate. Like this...
So if your plate is so far forward that you can't see the centre of it from a 45? angle on the other side its illegal. The plate doesn't have to be very far forward for this, just a couple of inches, near the axle being too far usually.
Basically they want the plate somewhere near to being in-line with the rear-most point of the bike. So an offset isn't a problem, its sticking it too far forward that will get you pinched.