Duncan,
Often your moms old tricks work best. The amount of heat you would require would almost certainly damage something, however tapping the lens in the right place might just free it up. Like mom did to free up Jar Lids.
Without seeing what your dealing with I can only give general instructions, so you may have to modify them to work in your situation... but Since I wouldn't want to tap on the glass it's self, or on any threads I'd try looking for a short piece of pipe, or a socket (as in socket tool set) it fits neatly into without the threads or glass touching anything.
Find one for the top as well, however you don't want them touching in the middle
Gran out the old hammer, Brass works best. Then give the top socket a couple of good solid taps.