I'm still of the opinion that its mechanical
I know you said Dave has checked the tappets, but if one or both of the valves are not seating then your symptoms would make sense.
Its not a long job to take the head off and check them for leaking.
Just turn it upside down (the head) and fill the combustion space with petrol, if it stays there they could be ok.
If it dissapears into the ports then regrinding is the cure.
Personnally I would regrind while the head is off anyway just to be sure.
Of course I could be wrong (perish the thought) but its not "major surgery" and something which requires only basic tools and knowledge.
Taking it to a dealer is a "last resort" remedy, as workshop time is very expensive.
I think you've spent enough on it already Stella, time for some good old fashioned Yorkshire common sense.
These bikes are sold worldwide and kept running on a shoestring by people far less educated than us. Basic principals are very simple and apply to all engines no matter what configuration from singles to multi's they all require "compression" to work effeciently.
I'm off the road till probably Wednesday but I will be at Squires by hook or by XK, so if someone could get it there I could have a look at it and it could possibly be ridden home.
Let me know if this option is required as I dont want to bring all the grinding stuff if I wont need it.
On the plus side, it might be quite instructional, for some, to see this once common practice actually carried out.
The pioneers were regularly seen at the side of the road "de - coking" and regrinding valves. The materials were inferior in those days and didn't last very long, and the oils and fuels left a lot of heavy deposits in and around the combustion space,ports,valve heads and stems and exhausts.
The bikes would become ever more sluggish until they would ultimately refuse to start even with a *bump*
Then it was out with the spanners and fix it cos there were precious few "dealers" to turn to.