Congratulations PPL32,
Well done for passing the exam!
ISA temperature at sea level is 15degrees C, In ISA you lose 2 degrees C per 1000ft, therefore in ISA it will be 7 degrees C - that part does sound right to me (it has been a while since I have done this). But that is only a small portion of the take-off distance calculations, weight, type of runway, long grass vs short grass, is it ground roll distance, or distance over 50ft, could you have misread the question, or used the wrong chart, or miscalculated the headwind component or forgot to apply the 0.33 (safety factor) that you need to add over the distance you obtained?
I would ask the examiner, or the instructor for a breakdown with regards to why you didn't get full marks on my next visit. Then hopefully you will learn from it!
Congratulations & Safe Flying!