Chaps,
Thanks to all of you who provided solutions and suggestions. In my case because the lock barrel was simply rotating it was not possible to unlock it conventionally, no matter how we fiddled with the lid and hinges. Similarly, I could not find the 'oles and pin mentioned by Chuck Donahue, so off to the locksmith we went.
He tried to remove the lock barrel elegantly by drilling a small hole and releasing some locking lever in there, but that didn't work so it was back to brute force. He then drilled out some of the lock cylinder (without getting metal in the tank, because he didn't drill right through the latch mechanism), and removed the cylinder.
As Chuck points out, there are two pins cast as part of the lock barrel, and one of these had sheared off. This allowed the lock to rotate 360 degrees, and because one pin was missing, turning the lock didn't turn the actuating mechanism, a cam which slides the latch tongue back. Putting two small diameter Allan keys (hex wrenches to North Americans?) into the holes, and placing a large, flat bladed screwdriver between them rotated the actuating mechanism enough to slide the latch tongue back and allowed us to open the cap. If you are caught in a similar situation, the screwdriver had a blade about 13 mm or half an inch across, and was inserted *vertically* between the pins; turning the screwdriver as if undoing a right-hand thread applies pressure to the Allan keys down low where they enter the holes in the actuating mechanism. All of this took about thirty minutes, and set me back thirty pictures of the Queen, which I thought was good value.
Three Phillips-head screws inside the cap allowed me to remove the actuating mechanism, which is in three major parts: a) the circular plastic body, b) the latch tongue, which slides horizontally across the bottom of the circular body, and c) the "cam body", which is a plastic cylinder. This cylinder is closed at one end, with a small snail cam moulded onto the outside surface of the closed end. Inside the closed end are the two holes into which the pins on the lock barrel mate, and the lock barrel is a neat sliding fit inside the cam body; turning the lock turns the cam body and the cam makes the latch tongue slide backwards. If you are tempted to disassemble this mechanism, be aware that there is a coiled compression spring inside the latch tongue to force said tongue into the cap-locked position when the cam is released. There is also a small circular snail-type spring under the cam body which forces the cam body and lock barrel to return to the tank-locked position when key pressure is released. If you pull it apart, I encourage you to do so under a piece of clean cloth so that you can recover the springs easily, and not spend half an hour crawling short-sightedly around the workshop floor, muttering imprecations whilst searching for very small springs.
If you need to find parts may I suggest that you look at
http://www.munichmotorcycles.com.au; they seem to have lotsa parts of all sorts, and their website is easy to navigate. Note that I found that it is possible to purchase just the lock barrel, for AUD$35-00, which seems good to me; see this link -
http://www.munichmotorcycles.com.au/product_info.php?products_id=1876So now I can, and have, put fuel in the tank and go for a ride. Great 22 degrees C weather, sunshine, and a wasp in the eye (sigh). Ah well, every silver lining has a bloody big dark cloud wrapped around it.
Greg