The address on the website is:

33/35 Daws Lane
London
NW7 4SD

A search of businesses at this address on 192.com gives quite a few and they all appear to start with 020 8906 6xxx. Perhaps the office and phone system is operated by a third party.  

I notice that Office Front is based there, don't know whether that's got anything to do with it.  

So I suggest that the number you are looking for starts with 020 8906 6xxx. You could call the 0870 number when they are closed to see if it answers with the extension number. You could also try the 0870 number on the Creative website 0870 919 8000. If you get an extension number you can then probably put that on the end of the prefix I suggested.