Like the other guys here, I would say replace the whole thing. Just pouring on top of the old surface is asking for trouble. You will never get a permanent bond between the old slab and the new one. In ten years or less, (ice) water, plant roots and bugs will find their way into that seam and just break it right apart.
Sledge hammer is a real beast - I work out and go to the gym, and even for me using a sledge to break up my concrete steps a few years ago was pure hell. The electric jackhammers really don't shake up too much - they just break the stone under them. So no need to worry about your foundation.
If you can help it, get a jackhammer somewhere. Down here in the States they are easy to rent. I actually had a brother-in-law that had one in his garage, just sitting there. It makes a WORLD of difference, though.
Look in your area for a, "Ready Mix Concrete" company. There are companies out there who will bring a special truck to your site, and they mix the dry ingredients right there on the spot. There is a meter on the truck, and you only pay for what you use. Some places may have a minimum order, like a cubic yard, so plan out other areas where you might need the concrete, or maybe make a few artificial boulders with the leftover stuff