Although I can't measure my belt deflection right now as the car is at the dealer awaiting ECU....

Q: Has the noise got worse over time (or constant)?
A: Has reduced somewhat over time. It is almost silent when the oil is cold, but noisy after a decent fast run or 30 mins of driving. (Best 'fix' is earplugs though...)

Q: What tension have you got the belt at?
A: Probably 15-20mm mid-way on lower run.

Q: Is the belt alignment true (as far as you can tell)?
A: Yes, absolutely. One of the the few issues I've not had.