Resolved... (watch this space :)
Cheers Gollo. Although it looks to be resolved now... I borrowed a PCMCIA NIC and dongle and this different card (with exactly the same settings as before) works fine. I now get a reasonable download speed (between 15 and 20 kbps) and the download doesn't cut off anymore. Just to confirm, I then swicthed back to the onboard card (with exactly same settings) and tried again - back to old, terrible connection and downloads just not happening.
Not sure if it's related to some extra configuration that I don't know about (since there seems to be a whole load of advanced features when compared to my PCMCIA card) - or perhaps it just doesn't like static IP ?? (I spoke to another guy at work who has same laptop and he reports a similar issue when using static IP).
Cheers to everyone for all the help!
Dan.