It is important that the new small extended section of the"concrete" has good bonding with the other parts. Else, with heavy load, it could just give way. This will not happen immediately as there are tiles to temporary support, but with time and load, it will start to crack and drop.
You could ask your contractor to provide metal supports across to the base of the hob. Spanning from the back to the front. However, will need to check so that it does not interfere with your kitchen cabinet door.

This post has been edited by kelvyn: Nov 28 2011, 11:20 AM