Managed to fix it - actually was a 7680. Only do this if you:
1. have a small screwdriver set
2. are handy putting little delicate plastic pieces and springs together again
3. are prepared to face the possibility that you might break the shaver entirely
4. you are out of warranty so you don't care if you invalidate it
THE PROBLEM is caused by a small flat metal assembly that acts as a spring to hold the blue button in place. That is held by 4 small glue posts. The glue breaks down, the flat spring falls out of place and the blue button is no long in the right place.
TO OPEN UP THE SHAVER
a. gently remove the plastic cover around the electrical socket underneath, using a small flathead screwdriver to pry it open
b. the two metallic sides of the shaver can be pulled away starting at the bottom of the shaver
c. uncrew the four screws on the underside of the shaver
d. remove cutter block, battery/lcd block (make sure you don't lose the grommet around the
lcd display
e. pull the spring and little plastic latch off the underside of the switch, make sure you remember how to put it back (took me almost 20 mins to figure out how to get that back afterwards)
f. slide the switch all the way up and pull it off the slide - you might need a little gentle force and a flathead screwdriver to coax it off - be careful you don't break anything here
g. remove the trimmer from the rest of the switch
h. put the blue piece back in place (mine had fallen out), then glue the flat metal spring back in place. I smeared brush-on crazy glue on the flat surfaces to hold it in place, I would imagine a four drops of reasonably strong glue on each of the glue posts would work best. Wait for the glue to dry and achieve strength.
i. slide the trimmer back into place, then the plastic latch and spring - this is the tricky part. Make sure the switch can slide on/off in all positions properly.
j. put the display/battery & cutter block back in place. put the back of the shaver on, hold the shaver together with your fingers and test the switch works in all positions without jamming. If so put the four screws back, followed by the metallic sides followed by the plastic cover of the electrical socket.
GOOD LUCK!!
