When I turn the key to the "ON" position, the temp gauge flicks up a bit, to just below 40C. Any idea what it should go up to if the block heater is working properly? Block heater has been plugged in all night (longer, actually).

Before I turn the key, the needle is all the way down...so it does move a bit.

According to the previous owner the glow plugs were replaced in the summer of 2006, so they're not even 1 year old. The battery was replaced right before the glow plugs.