This place usually comes up trumps, so...
If I were to purchase a new sim-free, unlocked mobile phone and wanted to go back to Virgin Mobile from Three, would I need to purchase a new Virgin Mobile SIM card...
...or, is it in any way possible re-activating the Virgin Mobile SIM card I used to use and still have?
If this information is necessary, I did take my old Virgin Mobile number with me to Three and intend to bring it back with me. I presume therefore that since the SIM card is no longer "tied" to anything, I will need to buy a new SIM card and put my existing number on to that?
SIM cards, PAC codes... what the PUK?
In theory, as long as you have the old simcard and you didn't do anything odd like changing your old number you can pop it in and away you go.
edit: Ok so I hit reply before reading it all. Yup you'll need to get a new sim then ask for the number to be transfered to it.
edit: Ok so I hit reply before reading it all. Yup you'll need to get a new sim then ask for the number to be transfered to it.

As I understand it yes, you need a 'new' virgin number, so a new sim, then you can port your number back to them, which will be put on this new sim.
I have moved mine backwards and forwards between orange and t-mobile without hitch in similar circumstances.
I have moved mine backwards and forwards between orange and t-mobile without hitch in similar circumstances.