First bill

[#1]

So I joined up on Sunday and the first payment has been taken from my bank account even though I won't be connected until 24th April????

[#2]

That is normal
Although your first payment has been taken the clock won't start running until you are connected
Typo corrected after sarcastic remark 

[#3]

So my bill will be on the 24th of each month?

[#4]

Correct although it gets a bit complicated when you are getting phone and broadband and they don't happen on the same day
In that case - and I think I remember correctly - the phone part is part billed to move the whole billing date to the broadband activation date (this also happens in reverse as the broadband billing takes precedence so where you sigh up for phone later a part month is taken for the phone)

[#5]

Yes, if the phone is activated mid-month the payment is pro-rata'd so it matches up with the broadband payments