by chromedome on Sun 13 Jul 2014, 7:05 pm
It's a cable operated lock mechanism under the saddle, consisting of a 6mm steel hoop on the saddle base which engages in a slot in the frame, where there's a sliding locking bar which is in the unlocked position when pulled to the left.
I've just been out to the garage to look at mine, and the frame plating round the lock prevents you getting to the cable entirely.
Perhaps the inner cable has snapped or rusted into place. Alternatively, it's not unknown for something to jam in the lock (like a set of light waterproof trousers).
Only way I can think of to get to it is to lever the plastic saddle base upwards on the left hand side just above the lock barrel, and try using a screwdriver with a 90 degree bend (approximately 75mm from the tip) to get into the slot (located midships about 25 mm to the rear of the lock barrel) from above, and bear down/pull the locking bar back towards the left hand side.The taper on this bar is on top, so you might get some purchase. The problem with this is that you might damage the saddle base in the process.