In the interest of completion......
I stripped down the top end again. (amazing that it literally only takes 5 minutes)
Replaced the bevel gears with a spare known good ones, top bevel self aligning bearing and checked for any play and proper mesh on the bottom bevels.
Embarrassingly (for a professional engineer) and in the interest of honesty I found that the intake rocker clearance was massive..!!!
They are the original ball roller rockers with a flat on them and I guess the ball had moved as I fitted them to allow the round part to push on the valve and NOT the flat.... So after start the ball revolved around and opened up the gap as the flat then impacted the valve stem..... duh
Anyway the motor is much quieter than before and listening to plenty of youtube videos of various bevel singles seems similar... i.e. NOISY.
Thanks for all your advice.