We registered with TPA and that made a huge difference, oversea ones still come through, but I have noticed since I was searching online for insurance, I gave my number as 01000 000000 they have stopped completely.
Ensure you are registered with the Telephone Preference Service (TPS) http://www.tpsonline.org.uk/
After 28 days, this filter should be active and anyone calling after that time is breaking the law, or you may have consented to them calling you (via a survey, webpage or some other form). Name and shame the firm on here if they continue to harass you.