7syns over is nothing really. I know its supposed to be planned in advance but slimming world promote their flexi syns days, so you can have a night out or something without feeling guilty and compensating before and after. Very much the "draw a line and move on" kinda way that you get straight back on plan afterwards.
They say to expect a maintain or a small gain on a week where you've used a flexi syns day, but to be honest at 7 over, I really don't think you need to worry too much! I had a flexi syns night (35 on a meal and drinks) once and still had a 1lb loss so anything is possible! Obviously it may not be the same for you, but if you were on plan before and again right afterwards, I think you'll be fine xx