iPod 4th Generation or Photo Battery
All went quite easily, once got the case open. I only had two large iPod opening tools, and found the trick was to insert one quite quite forcefully (near the earphone jack as described) with the curved tip down towards the metal case and then it would stay in place - as it had inserted past the thickest part to the tip. Then I could easily slide around to the corner and insert the second opening tool on the long side. The adhesive holding the battery in place is very strong, but again using the opening tool I could insert from the side and underneath the battery and use it to lever the battery free.
The old battery was really swollen (twice as thick as the new replacement) which explained why the center button had become almost impossible to click, and of course, why the iPod had stopped working - it was unable to charge the battery.
As soon as I plugged the iPod into the charger it came to life, and now I have access to music that I have not listened to for 2 years.