BT today confirmed that the cap had been set correctly and it also confirmed that this is an error at their end. The £37 will be taken off my account when the next bill is produced.
This post should not be marked solved - BT have not given me any solution as yet
I am sorry that this has not been sorted out for you as of yet. @OlgaC dropped you an email on Saturday morning to let you know that she has sent a request to the mobile team to have this looked into. I have just doubled checked and your complaint is in the work stack for the mobile team and I would hope you will get a reply sometime today. Rest assured this will be resolved.
Sorry for the delay.
This is the response I received
I am so sorry that this hasn’t been resolved yet. I spoke with the BT Mobile team and they have advised that this is a known issue that is affecting a small number of users wherein the system is saying that your data usage limit has been reached even though it hasn’t. The BT Mobile team are working hard to resolve this but a fix may not be in place until the end of July. They have advised that you should increase your data cap to allow you to be able to use your service fully. Any excess charges will be refunded back to you. The BT Mobile team will check your account after your next bill is produced on the 20/06 and refund any excess charges that have been applied incorrectly.
So, no solution as yet - and raising the cap as suggested seems rather counterintuitive as it's actually in place for a reason. And the problem isn't the cap anyway, the problem is I've been overcharged for data while there is still a significant amount of data that is available within the allowance so raising the cap would not actually have the desired effect other than run the risk of BT overcharging by even more...
ah well, now it's gone further above the cap - it's a shame BT can't make any significant effort to fix something which is almost certainly a trivial fix in terms of programming - can anyone let me know if this deliberate overcharging (I use deliberate as this is now clearly over the cap I have in place and these charges have steadily increased since alerting BT to the problem) constitutes a breach of contract on the part of BT?
...and still the costs keep rising, I guess customers are just not important enough to BT any more to make any real effort to try and resolve their issues...
Thought I'd update this in case others are having the same problem - and to make BT aware that the same problem has occurred this charge period - what has happened is that BT have removed the excess charge from the most recent bill. But immediately the next period started the charges appeared again even although there is plenty of data allowance available. Hopefully BT will respond and assure me that this charge will be taken from my next bill...