Well done, and thank you all.

My apologies for mistake in my original post. The guitar is actually an F-340, manufactured in Indonesia. The date is unknown. I made the error because the shape and dimensions are similar to an FG-180 owned by a friend.

For a short-term solution, I am off to purchase some plastic wood, cocktail sticks and glue. (Do hardware shop owners really need to know what goes on in my house ?)

For the long term; the tuning machines appear to be as Sandy MacLean describes above. They are not really satisfactory; even the four which are firmly in place on the headstock rattle a bit. I will probably get someone with a proper workshop and skills to replace all six with standard heads.

Please bear in mind that the guitar cost me £60 in the first place, but has lovely tone and suits me perfectly.