If anyone else needs to take one apart, I managed to get the faulty one apart in one piece without breaking anything, I think
Once you've unscrewed the top and base caps, the base cap is still attached by a black wire, just position it so that the square part is placed in the end against the battery compartment, it's kind of sideways but it but it will fit.
Before you go pushing on it, there's the oled screen to think of, the board moves back and forth and you can see the screen moving, what I didn't notice until afterwards was that the screen sits in a small recess, if you take a look at the top end, with a jewellers loupe or magnifying glass, the oled screen is stuck to 2 bits of hard foam, but the pcb it's on is fairly flexible, so hold the unit so that the buttons are facing the floor and with one hand put a little bit of tension on the board with 1 finger, pushing/pulling it towards the filling chamber, I don't think it needs to be much, about 1mm, maybe less, then you should be able to push the whole lot out from the bottom in one piece, just be gentle with the bottom cap that's still attached until it's positioned correctly.
I can think I see why this unit failed
I've checked my faulty unit against the
@smokey_da_vape video, the oled side of the PCB looks absolutely fine but once you take the 4 screws out to look at the back of the board it's obvious, there's a big 4R7/4.7Ohm resistor, which you can see behind the red wire at 14s in smokey's vid, in the vid it's clean green pcb with the white silkscreen all the way down to the 2 chips to the right, on my board it looks like it's a ground pour covered in solder that's actually shorted across 3 pins on the first chip, the 2nd chip looks ok, 3rd and 4th chips have a similar area between them that is shorting 4 pins on one chip to 2 pins on the other, smokey also has one less chip than me on the back of the pcb. I'll post pics if anyone is interested.
I've googled the chip numbers for the shorted chips and I can't find any datasheets for them, they're marked 6504 GA5G2U and have 8 pins. I can't really understand how it managed to power on let alone work once, maybe those pins are all earth and they're ok to short and provide some heat sinking, the ICs in question are on the power side of the board and look like they're probably voltage regulators. I haven't had a good look at the oled side of the board but the main mcu (microcontroller) is an stm32F030C8, 48Mhz Arm Cortex-M0 value chip with 64KB of flash and 4KB of sram, 48 pin.