PeterH, I'm little bit confused that you say we don't need accel, actually without filtering, if using only gyro output, the big drift I'd have and change of set-point that happens, I get nothing, I tried it long before this experiment!
It is impossible in my practice to not to combine output of gyro and accel, and yet balance the bot! I've read so many works (of other people already done it) and everybody is doing it this way, writing that impossible to use only gyro!
Of course, I'm in no way on an authority to claim anything in control, just I report what I've read. Actually if you personally can do it, is because of your skills that I unfortunately don't have, all i have is a background in math and a lot of reading online about this project, and still with a bot that can't balance really 