Betfred actually pay treble the odds for just the one winner, porkchop. however, they no longer guarantee best price.
When i put my L15s on, i always do a £1 e.w. acca with the 4 selections and i usually put the e.w. component of the bet on with Sky as they regularly pay extra places (slightly lesser prices) and the win component on with 365.
Ladbrokes pay double the odds for one winner, and a 10% bonus for all 4 selections winning, but their accumulative prices nearly always appear to be lower than those at 365. I have done a £1 Patent, £1.50 e.w. Treble today and the odds @ 365 are 6/1, 4/1 and 9/2. Ladbrokes 6/1, 4/1 and 7/2 for the same 3 horses - a difference of almost £40 on the Patent alone, were all 3 to win.
I have never really dealt with any of the other available firms such as Betfair, Betway, etc, so i can only comment on those i have dealings with. You really need to shop around, porkchop.