Thanks for the replies!
The problem I was describing is what Larry mentions, it seems that the whole carrying arm moves slightly. Also I have found that tightening the largest locking screw with some force can help somewhat, but that is not ideal. It is an old piece of equipment, it probably could do with stripping down and cleaning.
I think also what Drew mentions is related, that is if I am right in thinking that Drew is describing the guide rail that is attached to the main column, it does appear to move slightly when the baseboard moves as I have described. However, it seems that the screws which attach it to the column are not adjustable since there is apparently no screw head (just a flat piece, rather like the bottom of a flat screw) which is odd, I will have to have another look when I get home tomorrow.
Agx, I am glad you have drawn my attention to those bolts underneath the baseboard, I do think they are adjustable now that I think about it which is handy because the board is slightly out of alignment, to compensate so far I have been rather crudely shimmying the easel with paper, but now I can sort it out correctly.
bluejeh, that is very kind, do you have the service manual by any chance?
Thanks for all your help,