Thanks all for the tips!
- Coil was new Bosch 6V approx. five years ago, so unlikely to go but will check (will not be before the end of this week).
- In-line disposable fuel filter is changed regularly, but will check again.
Other options:
- What about a bad condenser?
- Distributor cap: arcing inside the bakelite parts?
Methinks the distributor has to come off for an inspection.
Hanno