Michael, I have checked for all the high speed pivots being 'loose', there is axial movement and the new main spring bush is not tight on the spindle. I used to think of clocks as precision machines but they are quite different from model engineering! I'll look again at the pivots towards the balance wheel for the problem.
Phill, I did remember to lubricate the spring and pivots (second time around) and no I didn't use WD40.
Richard, While I appreciate your sentiments, as this was once Mother-in-law's I daren't throw it away!
John