OK, my witch uses this talent a lot, generally to be able to push the Personal Augury power. Reading the talent closely, it says "Any effects (of perils/phenomena) that would normally affect the psyker instead cause the body to burn..."

Origionally I interpreted this as any phenomena or peril. Now while every peril affects the psyker directly (except possibly Dark Summoning), the same is not true of phenomena. So my question is, should a phenomena such as breaking all mirrors withing 14 meters make the victims body explode or not?

No, it is clear on what happens for a phenomena roll. The body burns for 1d10 E damage that has a range of 3 meters..

I disagree, it's open to discussion.

Full text quote of the relevant passage, identical in both sections this is described: "In addition, should any Psychic Phenomena or Perils of the Warp occur, the effects are focussed through the dying victim. Any effects that would normally effect the psyker instead cause the body to burn to ash in a blaze of irridescent warp flame..."

The way I am now reading the above it states that the effect has to effect the psyker to cause the burn. So, I would suggest that a phenomena of Distorted Reflections will shatter mirrors within a radius centered on the victim, as that phenomena has no effect on the psyker. The next result up on the table, Breath Leach, would cause the burn-off as it does effect the psyker.

And before anyone cries munchkin, lets face it, 1d10 damage for a peril with both armour and toughness to soak it, not to many psykers will care about that after they acquire ceremonial carapace armour. Heck, if it is applied to the Dark Summoning peril (the peril itself has no direct effect on the psyker afterall), which would you prefer, 1d10 damage with the warp weapon quality, or a Bloodletter intent on eviscerating you...

And if we want to debate further about this talent, is it a +0 or -10 Forbidden Lore: Warp roll to do, as the two descriptions of it disagree on this point. Putting my GM hat on, I would go with the -10 as it's leangthy prep time means that talents like foresight and assists from PC's / Minions will be much easier to get than if it was a talent that you would use in a raging combat. And I would suggest as well (GM hat again) a house rule that the skill roll for the sacrifice is only made after the psyker chooses at what level he will use his power. No wimping out and suddenly fettering a power because the powers that be found your sacrifice unnaceptable!