If not, reach out to O2 on social media, links below this message, or phone numbers in here if you rather try calling: Guide: Coronavirus Community Help and Support
Edited to add: let HMRC know as well, as per this answer in HMRC help pages:
This is an ongoing issue with O2 and they need to sort it out.
You may have a zero spend cap applied to your account and need to speak to customer services
All numbers are in this guide Guide: Coronavirus Community Help and Support
or via social media
*The Game Is On*
Some have had success by turning off wifi before expecting the code to come through.
Yes MI5's advice to turn off the WIFI worked. I had phoned O2 on Sunday morning and spoke to someone who didn't know who HMRC were! Eventually he said he had lifted a block and to wait 24hours, which I did and it still didn't work. Fortunately by then MI5's solution came in
Good to hear @Joe17 👍