Hi C&L
I'm afraid they decided to be difficult and not tell you, but you can easily work it out by looking at the limited-capacity for work test in the ESA mental health guide in the members' area.
For example, if your medical report stated:-
Takes one and a half times the length of time it would take a person without any form of mental disablement to successfully complete an everyday task with which the claimant is familiar
you know (from page 35 of the guide) that that is worth 6 points.
As you were awarded ESA you must have scored 15 points at the very least. You need to study the guides and your medical report. There will almost certainly be an activity that you didn't score points for that you feel you should have, as ATOS are very tight-fisted with regards to awarding points. So you will need to pay particular attention to giving detailed and accurate information about these activity(s).
Good luck
Derek