Thanks for the comments everyone.
I re-flashed the firmware and also reloaded the Shuriken settings bundle and it hasn't changed anything.
Meanwhile, the high E string has also now stopped working in variax mode too.
I have rung the shop where I bought it and they have asked me to bring it in so their guitar tech can take a look at it.
Clarification just FYI, I found that the dead strings do work, it's just that the volume level is really low. The report I wrote before about there being no attack and then a smeared distant sound was just a result of the FX I was running at the time. If I use no FX and just turn the volume way up I can hear the string and the sound it makes is normal, but it's only 10% the volume of the other strings that work properly.
Bummer, I waited months for this thing to arrive and now it has to go back. :(