According to the BT Internet web site today, "From midnight on Friday June 4, BT Internet will be offering weekend access using an 0800 number for Plan Unlimited customers". BT will be making a new Windows dialling program available on its website that switches between the normal local-rate number and the new uncharged number: other platforms will have to make their own arrangements.
Plan Unlimited costs £11.75 a month, making the new scheme cheaper than Freeserve and other uncharged ISPs if you spend around nine hours or more online at weekends. You'll have to redial if you want to do this, however, as the service automatically cuts your connection after two hours of usage -- that's around 30MB at 33.6kbps speeeds. If you dial up but remain idle, the timeout is twenty minutes.
At the time of writing, it was unclear whether BT's ISDN access was supported under the new scheme.