If anyone is interested, I solved the problem the old fashioned way- discovered a NOS bronze bushing hidden at the back of the shelf along with some other goodies.
The replacement bushing currently being sold by Moss is not equal to the original and may not last very long as it is much softer brass.
Below are pics of the two bushings when reamed to size, the original is more copper coloured.
This steering box has been butchered in the past by a dpo hammering a balljoint splitter wedge between the box and arm to try and remove the arm. They also machined down the shaft to 25mm from 1" (25.4mm) and fitted an industrial bushing. This meant the tiny seal couldn't even reach the shaft and had no chance of working. Once the oil had escaped, the top bearing ran dry which ovalled out the steering box casting. To repair this I reamed the upper bearing back to round and had the shaft metal sprayed to build it up to the new bigger size. I also had the lower 25mm area built back up to 1" except where the seal runs. I then machined the battered seal groove out to take a 30*25*7mm seal which has a proper garter spring. With a NOS bushing, steering peg and bicycle bearings all discovered on the shelf it feels like brand new again.
Andy.
NOS Bronze bushing
Replacement brass bushing
Metal sprayed shaft
New seal