Hi djr_uk,
If you have an iPhone there is a code , *3001#12345#* , which if you enter via the phone key pad and tap send gives access to hidden information. In my case ( iPhone 7 ) I get a screen showing " Dashboard" above a Home symbol, tap the symbol on the right hand side ( 3 lines + dots ) and you should get "All Metrics", tap " Virgin" then "RAT" (serving cell info). The top line "PLMN" identifies the carrier, in my case 234 10 ( on Sim issued in July when I joined Virgin ), 2 is Europe, 34 is UK and 10 means O2, EE would be 30 or 33, Vodafone 15 and Three would be 20. Hopefully this will be helpful to customers who are unsure as to whether they have been transferred to O2 or not.
Regards,
Dave