Last night i had my first go at making 10 litres of home brew cider from the NZ Mad Millie's kit. It told me to store between 18 degrees and 28 degrees so I out it in my spare room. I woke up this morning and it had dropped to 12 degrees. Has this wrecked the fermentation progress, and will it still be ok??
No. It can take longer to ferment, but in general the slower colder fermentation will result in a better tasting cider. I have used a few cider yeasts that ferment at a hotter temperature and they tend to give a bit of a sulphur taste. Now even if I use that particular yeast, one the fermentation has started to roll, I drop the temperature down to the lowest temperature that the yeast will work at.
Slow and cold = better flavor
So assuming you just started this and this isn't in a carbonation/bottling part of the process...
No experience with your specific kit, but in general, a temperature drop like that won't ruin the fermentation. You just might need to kickstart it.
If your kit gives you a time frame in which the airlock should start bubbling, I'd wait 2 days past that. If you aren't getting fermentation bubbling by then, add a little more yeast and you should be back on track.
Fermenting too low will typically not hurt except that the yeast may go dormant and be troublesome to rouse or you just may not get the flavor characteristics from the yeast (esters, et al more common in higher fermentation temperatures). In any case, it should not "ruin" it, but may not produce all the desired flavor profiles or may need to be brought up to the correct temps get fermentation to complete.