Robbie, I had a similar problem with mine a couple of years back.
A big drift and a hammer will come in useful
, but the thing I did was to get a hold of some 12mm or 14mm threaded bar and some long bits of stout tubing, steam pipe is good.
I then used it as a draw bar, through the center of the pivot, to put lots of pressure on the pivot and then drift it out. Tightening it as the pivot moved.
It's not easy. It took me about 2 hours!!