Thanks.
I did renew the plugs and caps when the first misfire occurred and this cured it for a few miles.
I must admit to buying another (resprayed )tank off Ebay as the original has begun to leak around the prop area and NOT (foolishly cleaning it out before fitting it to the bike). BE WARNED AND NOT LAZY like me.
Yes the tank has now been flushed out several times.
I'm told it runs really well now and is extremely mechanically quiet -well it did with the borrowed igniter boxes. Hopefully the German ones when they arrive will be fine.
https://www.ebay.co.uk/itm/173945178487