I've used 1/4 odds but even so you missed the £5 return of the stake which would make the pot £140.75.
You should know as you are a betting chap that non handicap races only give you 1/5th of the odds.
To make it easy for the rest on here:
Pot was 138.75 less 10.00 (five quid each way) = pot now 128.75
Your horse did not win....so the first fiver is down the pan.
It came 2nd....so the last fiver goes on it at 1/5th of the odds (Starting Price 7/1 divided by 5 = 1.4/1)
multiply 1.4 x 5 =7 + the fiver back = 12
I forgot the stake and UM forgot the odds breakdown.... pot now 140.75.