OK, no idea what happened but as of tonight I'm actually getting broadband again, as opposed to speeds and pings considerably slower than my mobile phone. This is despite my modem power levels going even further out of spec (14.6dB Recieve level)...
Ping statistics for 64.233.183.104:
Packets: Sent = 100, Received = 100, Lost = 0 (0% loss)
Approximate round trip times in milli-seconds:
Minimum = 31ms, Maximum = 67ms, Average = 38ms
Four weeks since I reported it, but coincidentally also two days after I mentioned moving to a rival ISP on the newsgroups... Didn't expect network engineers to be working Saturdays night but either way, lets hope it sticks...
*Braces self for depressing return to 150kbps tomorrow evening*