Lister D timing gear play/ wobble

Hi all,
if you read one of my posts along time ago about my lister D knock you will have seen the included video but of those that have not here it is again. as you can hear the knock really is quite obvious and does spoil the nice running of the engine.
after talking to a member of this forum about our engines he alerted me to the timing gears as a source of the knock, I gave it a good shake and i found it to have plenty of play, the noise sounding very similar to the knock when running.
I was wondering what sort of play should there be in the large timing gear? I feel that mine is quite excessive.



thanks for any help given,
callum