I've just had a look on sky bet and this is what they say.
If you've had a bet on a football match and it gets postponed, we'll return your stake in full, unless the match kicks off before midnight (local time) on the same day - in which case your bet will stand.
If the postponed selection is part of a multiple bet, the bet will stand as a multiple on the remaining selections. For example, a fourfold bet becomes a treble, a treble becomes a double and so on.