Chris,
Someone on here put me onto a little device called "Call Blocker" somewhere around £50 I think. I was getting bombarded with calls from PPI, Windows Help, Charities, Accident Claims plus a load from home improvement companies. I was seriously considering changing my number and going Ex directory to stop these parasites from badgering me. Since I've had phone blocker it's been so peaceful...the phone will ring just once and the caller will hear a short message , as most calls are automated they just get cut off after the one ring. A genuine caller will be asked to press a number to get through....brilliant, problem solved .
Pete