OK, tried to bleed it without success so took off the top bolt to see if the oil was passing through the damper rod, which it was.
I then dismantled the top bolt and one ball bearing fell out when I removed the damping screw.
On inspection there appears to be holes for two, so could this be the problem.
I've ordered some extra b/b's but in the meantime I had a go at putting the one I've got, back in and for the life of me I can't see how it can be done.
Any help and advice would be greatly appreciated.

Thought there was one one ball in the adjuster. Anyway, it/they sit in the hole with the ring spring over the top once the adjuster is assembled. When you rotate the brass needle the ball/s hops from from groove to groove on the brass needle and give the "clicker" function.

To set up the adjuster, wind the brass adjuster fully into the top nut assembly and then in six turns. Now screw the top nut assembly onto the rod until it lightly bottoms on the cartridge rod and tighten the lock nut. At this point the brass needle will be fully into the jet at the top of the rod and giving max damping. Wind the brass adjuster out a turn and then stroke the rode up and down, I usually do this without the fork spring fitted because of the extra rod movement it allows for bleeding the cartridge. Depending on how many turns in or out the brass adjuster is, the level of damping will change. With the brass adjuster screwed all the way in until lightly bottomed on the rod jet, the top nut assembly / rod should be very difficult to stroke either up or down.

If not then there is a problem within the cartridge unit, a valve has most likely become jammed open, or my instruction are not very good and the the top nut is assembled incorrectly. .

Hope this helps.