Ok, here's another one
The battery in Joy's trusty old Forerunner 305 went wallop a while ago. Apparently it's a relatively common problem.
Anyway after a bit of googling, it turns out that even though the unit is sealed, the battery is replaceable
So I bought a new battery on ebay (€15) and following the instructions in this video http://www.youtube.com/watch?v=8vQTHF76B6A
Bob's yer uncle!