The Sykes-Pickavant ball joint separator as modified. The bolt has to be screwed in so far for the MGB ball joints that it starts making an angle with the lower arm it is bearing on. Despite its heavy construction I could feel the bending forces building up and feared it might shatter, just inches from my face. So I built-up the end of the lower arm so that as the bolt was screwed in, and due to the angles involved moves slightly down the arm from the hinge, its business end is always pretty close to a right-angle with the bit of the arm it is bearing on.
May 2015: This type (66055000) is still available (at a price, around £25 from some suppliers but I have found one at £40!) but there are a number of others online at about £10. Some of these have a curved bottom arm, indicating that perhaps the bolt will always be square to the arm. Another has a recess so perhaps that will also be OK, but others have a straight arm so depending on the angle the two arms take up on your track-rod ends, they may or may not need modifying.
There are also so-called 'direct acting' types which has the bolt bearing down directly on the pin of the track-rod end rather than a lever action, which wouldn't have the above problem, however I've always felt the flange that fits between the TRE and the steering arm is pretty thin on that type. My scissors type has a 19mm jaw, but is much beefier, so it doesn't matter if it only grips the outer part of the steering arm eye. But the direct-acting type would need to be as snug to the pin as possible to get the greatest area of the flange under the steering arm eye to spread the load. I've measured the pin on a TRE at just under 17mm, so a 17mm one would be the one to go for, not the bigger 19 and 23mm types.