I doubt this will cure your problem. The mag moving on its platform is caused by the countersunk screws that secure the mag to the platform coming loose. A known common problem. The mag and platform should be removed as one and the problem addressed. Its such a pain to remove this that I'm sure you will do this only once!! The bolts, nuts and locking plate you refer to are to clamp the platform to the engine plates and serve as an adjustment to the mag chain tension. They should of course all be present and correct.

Ted,as Keith says, your problem more likely to involve the state of the mag platform you have. The one I found in the box of bits that formed the basis of my WD16H was similarly knackered and as no relacement to hand at the time it required a lot of work to true up and make serviceable. This included reaming out the badly worn central mounting lug and resleeving it to acheive a snug fit. There was a thread on this on the WD (M20)site about a year ago.

I did at the time replace the locking tab you mention, mainly for cosmetic reasons. Les at Russell Motors kindly produced a NOS one (part no 9089) for the princely sum of £1.50 incl VAT, that was January last year.

Should you wish to fabricate one, photo below of my old one from the scrap box, you are welcome to this if of any use but it's a bit tatty.

Thank you all for your tips ect ,

In the end I Went to my local tool station store and brought a box of three Three legged pullers £19
The small one was perfect for the Job !! the only one that i had in stock was big enough to pull the fly wheel off the Titanic
And by heck it didn't half make a bang when the mag sprocket popped of
while its off i will clean the commutator with a swob and thinners and reset the mag gap ect

Thanks all I will keep you posted TED

Sunny day so I PTFd the fuel tap threads and annealed the brass washers. No leaks, happy, until I noticed a small amount of sideways slop at the points end, between the mag and it's platform. The manual implies that one should probably remove the timing chest cover. I don't appear to have any vertical movement, never the less is this something I need to deal with directly: and is it best to remove the chest?

Well despite the underwhelmng response, I finally got round to sorting the loose platform. Have flu so can't work hard, or even think hard.
The engine plate bolts appear to have been tightened by someone more used to tractors.... After jury rigging extensions to various spanners/rings the magdyno came off intact, using a 40 year unused puller.
Contrary to all written advice I didn't remove the timing chest cover or even loosen it. I merely had to slip off the Dyno on refitting; after cleaning and loctiting the four screws, I used an impact for good measure. I initially scribed a mark for the points so I didn't have to retime the mag.
The biggest time waster was getting the nut back behind the clutch housing. I finally pushed a ring up with the captive nut and after tightening managed to slip the ring off, just....
Anyway it turned out my bike had a vertical slot in the back of the timing chest casting just behind the mag sprocket,thus negating the need to remove said casting.
Is this normal?

Yes that slot is present on all timing chests. Don't know where you read to remove the timing chest but it's not necessary, it is of course required to remove the timing chest cover.
Not so sure about the scribed line to keep the timing. As soon as you pull the mag chain sprocket off the timing is lost.