Woo hoo... Found a way to stop them.
As we've all discovered by now, ignoring the calls doesn't work. So, next, I tried just picking up and leaving the phone off-hook when they called. There was actually a lot of satisfaction to be gained knowing you were letting a company that is desperate enough to resort to auto-diallers know what it feels like to have to put up with silent calls.
Still the calls kept coming though, but it didn't feel quite so annoying now, as I knew I was costing that company money every time they called. (Insert Sid James style filthy laugh here...)
What finally stopped the calls once and for all though was just getting my computer to answer the phone when Caller ID showed they were telephoning yet again. When their auto-dialler registers that the call it has initiated has been answered and routes it to a member of their call-centre trash, they think the number is a fax machine and flag it accordingly. Result - no more calls.
You can do it easily enough by just installing some sort of fax software - they all have an option to manually receive a fax which causes your modem to pick up the call and start screaching. Having been pleased with the result, I've automated the process now for any other evil organisation that might choose to follow Weatherseal's appalling practices. PC World sell Zoom modems for 25 quid that also have UK caller id support. After knocking up a quick script, my computer now greets any phone numbers I've previously told it to with a fax-style tone automatically. Sorted.