You need one of these:
http://www.maplin.co.uk/Module.aspx?...ce=14&doy=1m11
There are two problems with an unswitched splitter. Firstly, there's a good chance there will be a frequency clash between the output of the cable box and the signals from the aerial, and second, there's a risk you could end up transmitting the output of your cable box from the aerial, which is a big no-no.
Or if you are in an area where VM provide digital service, another solution would be to switch to digital cable. You can then use SCART to connect the cable box.