I bought one these Universal Steering Knuckle Spreader Tool Removal Installation Ball Joints A6011 and have now used it 3 times highly recommended saves loads of time and broken screwdrivers. Once the clamp is spread the ball joint just drops out with a single light tap.

It also works on front struts that clamp to the swivle hub as on the Sierra, Montego, Rovers and lot of other cars.

Does look like a simple but clever tool - video guide - https://www.youtube.com/watch?v=YHZ4v8SB3OE

Probably better than the cold chisel and hammer method I've always used

I ground down an M12 bolt so it was just a hex head and a rectangular shaft. The rectangle had one dimension that was smaller than the gap, the other dimension slight larger. Insert the bolt into the gap, use a spanner to turn it ninety degrees, job done.

Grind a screwdriver shape tip onto the short end of a large allen key whack it in and turn to spread though I do like the special bolt idea too

Broken my fingers a few times with a chisel and hammer to split the clamp. Would much rather a low impact solution but never have the time to make it when I need it, and always forget when I don't need it! Looks useful but fairly large - not an issue getting into the cramped spaces ball joints are normally in?