Just tried the method
@SkulleateR used to get it working, by turning autologin off, installing everything but the avcodec and then re-installing avcodec after restart, still no go, still no playing mp4 in AmiBench and the only difference is now the system boots straight to the Amisphere login page.
It really looks the only way I would be able to fix this is with some sort of factory reset.
Who can I contact to enquire about doing this? Someone here, or should I email AmiKit directly about this?