The heading flown is always MAG unless flying polar routes. Does it change from Mag to True at a pre-defined point then... e.g above 60 North all headings are flown (and given) as True?
Absolutely correct. And yes, it changes at a specific latitude, and if my memory serves me correctly it is at 63 north but I'm not sure and could be wrong.
Winds are only mag near the runway (and then only round non-polar airports, I guess). Otherwise they are always given as true. I must confess, I thought most winds were measured from airports (ie. that's where the reporting stations for VOLMET etc were) - does this mean that surface wind is given in Mag degrees, but wind at altitude is given in degrees true?
Basically an airport wouldn't tell any aircraft the wind at altitude. They publish it to metoffices who compile the wind charts. And yes, they publish it to them in TRUE.
However, when landing or on a wind check, they will give the pilots the mag wind, since the pilot is thinking in mag and wants to know cross/headwind factors on the active runway (which is in mag as well). They measure this wind by a simple windmeter outside, which is aligned with magnorth.
I've never actually looked into this, but I think even the polar regions would still use mag headings for the runways and thus the surface winds, since even there on the approach the variation shouldn't change that much. However, I've not checked this and could be wrong.
If you're flying en-route (to repeat an earlier question), what heading is it that appears in the ops room via your ACARS system? Mag, true, or whatever is currently selected via your FMS?
We don't use ACARS, but I would say it reports whatever the FMS gives at that time, which would be mag unless in polar regions (above the specific latitude).