I have two quests, a dialogue quest and a combatant quest.
I'm trying to make it so that when finishing a bit of dialogue in the dialogue quest, a property from within a script (attached to the combatant quest) is modified. I've tried absolutely everything to get this damn thing to work and it's refusing to.

WHPMatchInProgress is declared as a property in WHPArenaSpectatorQuestScript.
This is what I've got at the moment:

WHPArenaSpectatorQuestScript Property WHPArenaSpectatorQuestScriptTemp AutoWHPArenaSpectatorQuestScriptTemp.WHPMatchInProgress = 1

Which produces this error:
[topic info 0E0033AD on quest WHPArenaDialogue (0E00338F)].WHP_TIF__020033AD.Fragment_0() - "WHP_TIF__020033AD.psc" Line 9
[08/02/2012 - 10:00:08PM] error: Cannot call WHPMatchInProgress() on a None object, aborting function call
stack:
[topic info 0E0033AD on quest WHPArenaDialogue (0E00338F)].WHP_TIF__020033AD.Fragment_0() - "WHP_TIF__020033AD.psc" Line 13

Please, someone put me out of my misery. This was so simple to do in Oblivion and it's driving me insane.

You need to hook the property up in the Creation Kit to the quest in question. It's reporting that it's a None object because WHPArenaSpectatorQuestScriptTemp has not been resolved. (Also, if that's a 1 / 0 Yes / No property, you should probably use a bool instead).

Well don't I look like an idiot.

Thank you, you've saved me from going prematurely bald from ripping my hair out. I take it property values (like linking up a property with an object) are saved in the plugin file as opposed to the script file?

Again, cheers. You're a lifesaver.

Yes. Properties are saved in the plugins.