I finally managed to get a new SIM card by managing to speak to somebody on 789. When the card arrived and I inserted it, I got a Phone Restricted message.
So as per the instructions on the letter that came with the SIM, I went to www.virginmedia.com/unlock and inputted the IMEI number and my info and was told I'd receive an email within 72 hours. 240 hours later, I've received nothing.
Now normally I don't mind waiting but I keep receiving texts from Virgin Mobile saying things like "URGENT insert your new SIM, all calls will stop in 10 days" etc. etc.
So I'm expecting to be cut off any day now but don't really know what to do if Virgin won't get back to me.