The fun didn't last long. The cartridge socket is held to the PCB with two tiny plastic tags, which have already failed. This meant that the 4 solder joints and thin copper tracks were taking the whole pressure of insertion. Needless to say, it ripped the tracks off the PCB. I repaired it last night and took a look at the buttons, there's not really enough space to put a decent solution inside, but maybe I need to look at it longer.
Interestingly, as mentioned in the video, the cartridges don't contain anything more than a few jumpers, no EPROM or whatever. All the games are in the device and the cartridge is just doing bank switching. The cartridge has 4 connections, GND and 3 inputs, but it only came with 2 cartridges. So shorting the third pin, may give you a whole new bank of games, but I haven't tried this yet (it was late).
Bryce.