Many ISPs automatically disconnect dial-up connections after 2 hours. It seems that Supanet is one of them, even though they can't be bothered to mention it in their Ts&Cs:
http://www.supanet.com/pdfs/dialup_terms_v2.pd f
It's unlikely that you'll persuade Supanet to override their system so that you can complete the large download. (With their support phone lines being charged at �1 per minute, it could cost you a lot just to find out that they can't help).
You'll either have to switch ISPs or, if the download is worth a small payment , use a pay-per-minute service on a guest basis.
To use a service which doesn't cut off after 2 hours, simply go to Dial Up Networking, and create a connection which dials 08456389800, with the username 'guest' and the password 'webguest'. (That's the guest log-in for Webtribe, which is now part of Freewire). 0845 numbers are only charged at 0.5p per minute at the evening rate. A typical dial-up download will get you about 10Mb per hour, so a 50Mb file would take around 5 hours and cost you �1.50
Chris