Either call in or web chat in about it. To call in, ring 150 from a VM phone / 0345 454 1111 from any phone -> enter the main menu -> option 4 -> go with either option 1 or option 5 (option 5 closes at 8PM weekday wise.) Let them know that the wrong STB type was sent out and ask them to get the correct type sent out, along with a collect+ sticker to use to return the 500GB model.
Or web chat in -> use the account and billing option -> My Broadband, TV and Home phone package -> Chat Online. All agents busy? reload the page every three minutes and check again. Access the page from a desktop/laptop. Between 8AM to 8PM weekday wise, 9AM to 5PM Sa, closed Su.
- - - - - - Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.