Breathemail were doing a £50 one-off and guaranteeing 24/7 for life. I don't know if they're still doing it, but initially they were heavily subscribed. Couple that with Skytalk at 40% less than BT line rates, free calls via mobile after 7pm, and my monthly BT bill has dropped to around £2! Now on low user tariff!
Hunt around - it's worth it.
Cheers