If attempting yourself... be careful not to use something with too small a diameter - it would leave a "corrugated" edge with waves in it.
If doing yourself invest in a bit of timber and turn it to the right diameter a little bigger than the inside diameter of the pipe and then taper it a bit on one end to be a slightly smaller to fit into the pipe. Do bent out the dent a bit with something like a braker bar or shovel handle, but make sure to keep that area smaller than the desired outside diameter (just a rough shaping to fit your block in). Then tap your shaping block into it and it will bring it back to the right diameter and perfectly round without dents or waves. Put a big screw or bold into the back of the block to remove easily as probably will be a wedged right. Materials shouldn't set you back much more than a fiver or so and will look right after.