It's in the footnotes of a tariff sheet pdf.

http://www2.savetodaysavetomorrow.com/documents/Online_v5_Ratecard.pdf

ADDiTiONAl CHARGES AND DiSCOUNTS

1. An electricity customer paying by monthly Direct Debit will receive a discount of £8.40 (£8.00 exc VAT) per annum. This is calculated daily and is credited to your electricity bill quarterly.

2. A gas customer paying by monthly Direct Debit will receive a discount of £16.80 (£16.00 exc VAT) per annum. This is calculated daily and is credited to your gas bill quarterly.

3. A Dual Fuel customer paying by monthly Direct Debit will receive a Dual Fuel discount of £25.20 (£24.00 exc VAT) per annum. This is calculated daily and is credited to your gas bill quarterly.

Maximum Discount available is for Dual Fuel on Direct Debit i.e. Electricity £8.40, (£8.00 exc VAT), Gas £16.80 (£16.00 exc VAT), Dual Fuel £25.20 (£24.00 exc VAT), Total £50.40, (£48.00 exc VAT).

(You are right that a '2% under our standard prices' is not much of a guarantee or assurance, though!)