In reply to Gibson:
The main thing is to give it time to heat through- so duration is worth more than 'hotness' per se.
You could, say, bring a vessel to the boil, turn off the stove and leave in the hot water for 5 mins. It's usually worth giving the bag a bit of a 'manipulate' half way too, to get the bits in the middle a bit of a 'stir'.
Used to love Corned Beef Hash, btw.
For some reason, many didn't.