I’ve learned a lot about RFI this past couple of days … The treadmill radiates RFI through the power cable, back into the power socket, and from there through the entire house’s ring main. Ring mains, a UK peculiarity invented after the war to save copper, are particularly susceptible to this.
You can easily hear the interference using an analogue radio and placing the radio around the house with the treadmill on and off. This page (currently not available) has some nice graphs from a radio frequency analyzer.
The solution was to purchase a power strip with a big choke and plug the treadmill (and nothing else) into it:
Although the lead from the treadmill to the power strip (and the treadmill triac itself) can still be heard on the radio emitting a lot of noise, it’s much better than having the entire ring main act as a huge antenna, and most importantly it no longer interrupts my ADSL line. Success!