why does my corsa 1200 sxi lose power on hills?

I agree. The most commong habit [my wife does the same] leave your foot resting on the clutch pedal once in top gear as you excellerate you'll unconsiosly put slight pressure on the pedal and that causes the plate to slip eventually wearing it out. Once in top gear get in to the habit of taking your foot off the pedal until you need to change gear again. Check with a garage it's the clutch.
Doubt it's engine as it would loose power on flat as well as on a hill. To Test clutch: start it up put it in 2nd gear with hand break on [if that's any good] and try pulling away if clutch is ok it should stall, if it doesn't stall and revs stop test and get the clutch replaced.

Bob
August 2010

When the clutch begins to fail you usually first notice it on hills - revs stay good but the vehicle slows down.

Peccavi
August 2010

It's most probably been caned to death during its life and the engine is now knackered.

Alexander the meerkat
August 2010