Hi Gareth, its all a matter of how much loss your talking about. If it starts at around 68 and drops to around 64 after an hour thats probably fine.
The truth is that you will make good beer over a range of temperatures of a few degrees, but unless everything is repetable you won't be able to make the same beer or guarantee exactly what will come out the end (i.e. one time it might come out full bodied the next drier with higher alcohol).
The next thing I will say is that most of your temperature loss is probably in the first 10 minutes and isn't really temperature loss so much as heat from the water goes to warm up the esky or pot or whatever. I don't know how quickly this would stabilise and it depends on what your using for a tun anyway.
As for opening it and adding heat, yes you definitely can. Some people add hot water to raise the temperature of the mash in a "step mash'. Some people also remove some of the grain and heat it in a kettle on the stove in a 'decoction mash'. You probably don't want to do either of these until you've read a bit about them and know why you would want to do them. If your just aiming for a constant temperature I'd leave it with the lid on and not touch it. Opening the lid looses too much heat
Finally the biggest thing I ever did to hit my mash temperatures more accurately was to stop putting the grain in the adding water and start by putting water in the tun (at 86C) and leaving it to cool until it hit 76C (give it a stir to help, my tun and manifold absorb most of that heat though) then adding grain to drop it to 66C which is usually my target.