I was driving along and car didn't feel as if it had full power,about 40 miles later car started losing power altogether and i heard a loud hiss of air from the foot well so i pulled in.Car would start again but not for long and was quite loud,now car doesn't start at all.Garage says there's low compression in 2 cylinders and it also took a while to get fuel to the engine,don't want to spend money taking the head off if its knackered.Please advise?