OK, after another week of trying most things within my limited technical ability -
New plugs
Fuel pump out - replaced filter, checked all hoses and connnections (all sound.)
Blown through breather pipe - all clear.
Added some injector cleaner to fuel but no difference.
No rust or rubbish in the tank.
It is now misfiring sometimes with a full tank - but it is still intermittent. Go for 70 miles without a hitch then just won't rev - feels like it's suffering from a kinked fuel hose or blocked breather - but all checked and all clear.
Starts perfectly and ticks over nicely - and may start off ok, but other times starts coughing and spluttering not long after setting off....
Any other ideas welcome (or I may throw in the towel and get someone who knows what they're doing to look at it
)