Hi Neil,
Just a thought...
The tube that fits in the frame head stock (begins with 'E' but I can never remember how to spell it..
) has a fixing pin which is located on the R/H side of the head stock at the top, this pin is designed to fix the 'E' tube in place and then the 2 head stock bolts (located at the front of the frame) can then be tightened.
This locating pin might be missing which could be causing you the slight movement problems...???
I will try and find a picture so you know what to look for....
999-FL