Don't use chemical metal to fill up the holes in the throttleplate.
Whilst working on the car yesterday I noticed it had begun to crack around the edge of both holes.
I removed the throttlebody and easily poked out the chemical metal - it doesnt have the strength for this kind of thing.
I couldnt be arsed to try it with the holes left open so I have blocked them off by putting a nut and bolt through and sawing off the extra bolt length.
Not a particularly cunning fix, but will work fine for me.
I am glad I caught it before the chemical metal dropped into the inlet manifold, and you can guess the rest...