Unless you get a sensible response from Nexgen, I suggest you start the Small Claims process against mGage Ltd. Tell them that you do not accept that any interaction between your phone and their service represents consent as it was probably the result of exploits on a web page or malware.
I tried calling 02037403517 and got a message to call back between 9:00am and 5:00pm Monday to Friday. It is not permissable under the rules to get you to call a premium rate number OR ANY NUMBER OUTSIDE THE UK.
mGage took the money on behalf of Nexgen, and they don't dispute that fact, so you don't need to prove negligence on their part. It IS worth scanning the forums of the networks and Twitter to find other cases similar to yours with Nexgen as the "service". You will find there are quite a few. Use these to point to the fact that yours is not an isolated case.
A sample letter before action can be found here: https://payforitsucks.co.uk/im-not-getting-anywhere-with-my-case/#letter
Let me know if I can help you further with this process.
As I have said, I completely agree with you that the attitude of GiffGaff is morally reprehensible and legally very dubious. They are aware that they are handing members money over to known scammers, and I believe that their failure to act to protect members amounts to negligence.
It is, of course, open to you to pursue GiffGaff in this matter. To do this you would have to show negligence. There are a significant number of complaints on this forum about Nexgen, and GiffGaff can hardly claim to be ignorant of the fact that there is a problem with this "service". It would be for a court to decide whether continuing to allow this company free access to members accounts amounts to negligence. I think it might.
There is nothing to stop you from starting a Small Claim with GiffGaff as defendant. If you choose this course, the payforitsucks website will do its best to support you. I think there is a very good chance that you would win.
However, in honesty, your claim against mGage is easier to provel as you have no need to allege and prove negligence.
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