Keep getting duff connections, where it connects but stops recieving at about 287bytes, usually leave on overnight d/loading, and this means it could sit there all night doing nothing.

Is there anything I can do to make the modem hang up and redial until it gets a worthwhile connection?

I use CiDial 2.3 to keep me connected. I'm not sure if will work with your problem but its worth a try as its a free program

What are your system specs?
What kind of modem are you using?
Have you tried to download new drivers for your modem?

Yeah tried new drivers, but its a new thing just started happening a few weeks back, think it's the ISP but at £1 per minute and sh*t service I'd prefer just reconnecting :)