Hello Mick,
Thnks for the suggestion.  Yes I have removed the earth wire - still dead. 
I had another look at it yesterday. When I went to it it was sparking like a good one. Put the plug in and tried to start - Dead!  Most frustration.  Tried manipulating the plug lead in case of hidden breaks in the wire but no difference.
Have remembered that it did this n me last year.  No spark and then suddenly it worked and worked the rest of the summer.  It has been working this year until recently.  It was put away fine and then wouldn't start next time out. 

The later OHV engine mag doesnt fit, the mounting holes are different.

https://www.ebay.co.uk/itm/384921479330?hash=item599f1ba8a2:g:~9cAAOSwnv5kgTng

Thinking of trying one of these and  seeing if I can modify either the mag or the mounting to make it fit

Assuming your engine is a Honda GV100? you should be able to get the correct coil for it.

The coil you mentioned is for a bigger engine and I would have thought the diameter of the flywheel would be different.

https://www.ebay.co.uk/itm/162538604782

this one looks right but nearly 120 pounds

https://www.mowers-online.co.uk/honda-gv100-engine-ignition-coil-assembly-30500-zl1-004

is £80  Where is the cheap chinese rip off copy when you want one

Any ideas who might be breaking mowers and have one?

Facebook Marketplace is a brilliant place to start.