Thanks again guys. Where would I be without you.
I thought maybe I had got lucky and turning it upside down had dislodged a foreign body but I took the bike out this morning and - I should be so lucky huh? Same old trouble but having a better idea how it works I could sense the clutch biting after a few blips of the throttle. Had to do this a few times before it started to behave. I think the difference is we had a frost last night and I guess there is grease that becomes solid at low temperatures. So it must all come apart. I had found Yosepower in UK and they asked me for the serial number. Can't see it at the moment so I guess it is either behind the freewheel or somewhere inside. I see I should be able to remove the screws without necessarily taking the freewheel off. Yosepower in UK appear to be in a bike shop about 10 miles away so maybe I can take the clutch there to pick up a replacement.
In videos I have seen the rollers and springs look as if they are meant to be clear of all grease but not accessible without some destruction. So maybe a good flush will do the trick once the clutch is removed. The gears clearly need grease which I take to be silicone I see can be had on Ebay, unless plumbers seal grease will also do for this.