Sorry I can't answer your direct question but I presume you have registered with the TPS.
If not you may find that this could cut out these pests. It takes 28 days for the service to become fully effective so if you haven't registered it's worth doing it now.
http://www.tpsonline.org.uk/tps/
I have just noticed on the TPS site that you can also register against recieving silent calls.
There is a separate service called Silent CallGard. You can register with them by calling 0844 3722325.
Off to do that myself.