Hi kcrose and welcome to the forums.
I've just double checked and £5.99 gives you 90 minutes to use with in 24 hours of first logging in
£6.00 per month for 500 minutes per month.
As you've only used 20 minutes, which I guess was within the 24 hours?, it might be wise to call the BTWifi team on 0800 022 33 22 (choose the 'BT Wi-fi' option).
They should be able to sort this out for you.