So, I had my first go with Zwift this morning.
I used the 25km free trial to do the full FTP test and it went ok but I'm not entirely convinced the power output was accurate as I'm not using a smart trainer.
I guess to a certain extent, as long as the setup is consistent, the actual numbers don't matter too much. You can compare 'wrong' figures with each other.
I also found that even though it was plugged in, my iPad was losing battery power, so if I do it again I'll need to make sure the thing is fully charged.
*Edit* I've just been reading up on it — my Elite trainer has five resistance levels. Apparently, Zwift's power calculations are based on this being set to level 3 — I always ride at level 5 because anything else doesn't provide enough resistance… looks like I'll have to change it to 3 and run the test again.