Yep, agree with the above. Worth noting, however, that A1 marks (accuracy) are for actually getting the right answer numerically and often subsume the M1 mark(s). B1 marks are independent and have to be scored by writing whatever that marking point says.
All in all, just write your working!
Doesn't take much time and reduces the chance of a silly mistake. Also stops you losing all the marks if you make an unwritten silly mistake!