Hi Kevin,
My 50 Comet was also very "tappety" when I got it running earlier this year.
I found very slight play in the Rocker bearings and on the advice of Coventry Section members installed the spares company's modified rocker feed bolts that pull the bearings up into the housings.
It has succeded in reducing the noise greatly, enough that now I can hear the whine from the timing gear!
Mike