Glad someone else can also benefit.
I have also noticed that the online billing says my monthly bill is £5 including mobile discount.
Think this should say £7.50 including discount (which was what I was paying).
So it suggests that whenever I do topup the mobile I MAY get another £2.50 discount !!!
My bill for last month was £4.62 which tends to suggest that they have applied the mobile discount AND my reduced monthly bill.
Anyway hope you also get your discount, I would give them a call as it is free and they are friendly and helpful.