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Thanks..... I will check those spindles out again.

any special tricks to do so.?

They should be tight in the holes in the head casting and need a threaded puller and tap out of the head. Then tapped back in again. (interference fit)
Also check the bronze bush and rocker pins for wear.

Valve seat loose?

OK...... top end all apart AGAIN..

I have a new double row bearing that I will replace anyway.
The rocker pins are nice and snug in the head, although they have a slight wear on the bushing surfaces and the bronze bushes have a bit of movement in the rocker themselves so I guess I will replace them.

the lower bevel has a light sideways movement probably less than 2 thou I didn't get the dial gauge on it yet.... Is any movement normal or acceptable.?

When wear is severe, it's worse on the steel spindle than the bronze bush.
I guess the case hardening isn't very deep.
When the hard skin is gone, no doubt wear will develop more quickly.
Wear of bushes and rocker spindles can be compensated for by adjusting the valve clearances.
I don't worry about them until they are markedly worn.

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.

Access to the top end is excellent on singles alright.

Thanks for admitting the cause of your noise problem.
A good reminder to check the simple things first, folks.