I have recently purchased a 1967 340. I need to replace the windscreen washer jets but cannot see any way to do this.
In additioon, there is some form of vent panel between the washers similar to that on my TR6. However unlike my TR6 I cannot find an operation lever to open the vent.
Can anyone provide me with some guidance on this item please.

 

the vent panel between the two washers is called the Scuttle Vent or Fresh Air Vent.

 

I did this recently on my S Type. It required removing the entire dash to access the jets. I had already don't that to replace the main wiring harness. It's a lot of work unfortunately.

The scuttle vent you mention is vacuum operated on the S Type. There is an actuator diaphragm above the Identification plate in the middle of the firewall behind the engine. That opens and closes the vent on my car and by pushing a button marked "Air" bellow the ashtray. If yours has the diaphragm then it is opened the same way but with a different switch. Some cars use a lever to open the vent but I think that was in earlier models. Here's a video on how to fix the vent if it is lever operated.

 

In a Mk2 (and so probably also in a 340), the scuttle vent is operated by a lever. On RHD cars, it's at the left hand end of the space under the centre dash panel. When the vent is open, the inlet to the heater from inside the car closes. Generally, the heater works better with the vent open as the external aerodynamics help the fan.

 

I tightened the loose jets in my S type but did not have to remove the entire dash, only the cap and lowered the hinged center panel for access. Was not very difficult. Maybe the 340 / MK-2 is different. Anyway,l it was a job that I enjoyed and learned a lot from it.