It probably wasn't one of your better ideas, but hopefully come WI day you might get away with it. Just try not to use any more syns today - eat as much superfree type food and do some exercise and the god of the scales might not punish you!
Biscuits are seriously dangerous! I have a thing about McVities Digestives, but I now just have one a day with my morning cuppa. If I found myself unable to cope with just one, then I'd have to stop buying them.
No way does this mean that you've ruined your week though so don't beat yourself up about it, just try not to get in the habit, because they are moreish little devils!