The standard Pace 4000 remote control uses an unusual protocol that won't work with most senders. The good news is the Pace 4000 will also accept remote control codes from the modern VM remotes, and if you can source one of these it will work with your sender.
This is the full range of VM remotes. Any of these except types 1, 4 and 6 will work with your sender and box.
http://help.virginmedia.com/system/s...MER_TYPE=Cable
ps. Welcome to the forum! And congratulations on still having a working Pace box