Hi, Cerridwen. I am not sure about HIIT impact on body fat but it certainly makes my running more enjoyable. I started running intervals some time ago and went from 40 mins slow and boring jogging to 20 mins that leave me exhilarated and gasping for breath. The one I do is from Bill Phillips's Body for Life (I've never done anything else from that book, it's hard core weighlifting, but I picked up the intervals and I've stuck with them).

The workout is based on Perceived Rate of Exhaustion (PRE). You start at 2 mins at level 5 PRE (it can be anything from walking to a brisk jog, depending on individual fitness level, for me it is a veeeery slow jog), then run 1 min at level 6, 1 min each at level 7, 8 and 9, then you slow down to 1 min at 6, then 7, 8, 9, then two more times 1 min each at 6, 7, 8, 9. The last time after 9 you go up to 10 (all-out running for 1 min), then slow to level 5 for a minute to cool down. End of workout - it takes 20 mins and leaves me with a great runner's high.

The other one I do from time to time to mix things up is to warm up with 5 mins of slow run, then do 30 secs at a very fast tempo, then 1 min at half the speed, rinse and repeat from anything between 10 and 20 mins. I found this in Runner's World magazine and modified it a bit (they go too technical with the description of speed - race pace minus one and similar stuff that makes my eyes glaze).

P.S. I run outside - not sure whether you can do this on a treadmill.