What exactly is this plug heat range?
Is it just a way to control/adjust ignition timing?

Is it possible to retared timing and run more compression to increase power and decrease chance of pre-ignition?

you're on the right track, read this, it has a glow plug guide
http://www.rcuniverse.com/forum/m_5517128/tm.htm

This is a simple question that really gets into the way our engines work. First thing is that the fuel (the methanol part) ignites because of a chemical reaction between the methanol and the platinum that's used in the plug coil. This is actually a catalytic reaction which just means that the platinum doesn't get used up in the reaction but stays as it is, in the coil. What the reaction does though is to put heat from the reaction back into the coil to keep it hot.

But two things are needed before this reaction can take place. First thing is the coil has to be hot enough which is why a battery is needed at first. The second thing is there has to be enough pressure of the air/fuel against the coil. This pressure packs more molecules of the mix against the platinum and this comes from the compression ratio of the engine. These two things, heat and pressure, combine to start the reaction. With more heat you need less pressure and with more pressure you need less heat. By combining these two variables you can set the point at which the mixture ignites, in other words it sets the ignition point.

Raising compression with the same heat range plug advances the ignition point but the ignition timing can be retarded back to where it was before by using a cooler plug. If compression is a bit low for the fuel being used then the timing can be advanced by using a hotter plug. This can only be done by trial and error but with an engine driving a constant load (on a dyno or driving a prop) then basically it's the plug that gives the highest revs because the closer you are to the perfect ignition timing the more power it'll give.

Just a minor point but we don't really have pre-ignition because (hopefully ) there aren't any glowing hot spots other than the plug. We can get detonation though if the ignition timing is too far advanced by using a combination of too much nitro, too much compression and too hot a plug.

Thanks for that,
But with too high of compression the petrol can pre-ignite by itself anyway as it heats up when compressed that much? (ie, like diesels work)
Is that an issues with these engines? Especially being 2 stroke and no valve overlap?

We don't have a problem with auto-ignition because the compressions used are far too low for that to happen. Most engines are around 9.5 to 10:1 with a very few up around 12:1 but they're designed for zero or very low nitro. Auto diesels run at about 22:1 compression to get enough heat to ignite the spray of fuel into the combustion chamber. The only engines that rely on heating up an air/fuel mix (which a true diesel doesn't do) is our model "diesels" but they need ether in the fuel because ether auto-ignites very easily. The burning ether then ignites the kero which is the part of the fuel that gives the power. Their compressions are around 17:1 but you can't pin down a figure for them because the compression is variable.