UPDATE:
Having discussed with the Webchat function (not sure if it was a human or a 'bot), it appears I have initiated the cancellation process already. I hope the below helps anyone in the same position as I.
Basically, if you wish to cancel a Data Add-on for a Mobile you have to initially go through the same process as if you are going to buy an Add-on.
Go to Manage Add-ons and make sure you select the correct Mobile if you have more than one on your plan, follow the instructions. When you get to the point where you are presented with a list of different Add-ons (500mb, 1GB, 10GB etc) you should note that the size Add-on you had purchased isn't listed. Scroll to the end where you should see 'Delete this Add-on'. This is the bit that throws you because you cannot select your size plan, it isn't there to select. Just click on 'Delete...'and 'Continue'. Follow the rest of the instructions to 'complete' your order which in effect is a cancellation.
This will show on 'Order Tracking' as 'Pending' , if not then try again.
The Webchat person/'bot assured me that two days before my next billing date I will receive an e-mail which will confirm the Add-on has been cancelled. I have copy/pasted my 'chat' for piece of mind and will be watching my bill like a hawk.
BT, you do not make it easy for anyone to understand this process and seeing as this should be a benefit to your customers who require temporary additional data for any number of reasons, it feels as if it is being done on purpose. I doubt it is actually, more a case of nobody capable/bothered to fix it. In the meantime you get disgruntled customers who give your customer service a bad name and waste the time of you staff/'bots. This same complaint has appeared on this forum for years.
