[5valve]

dont torque one side first. It has to be gradual criss cross patter. Grind a ring spanner or buy a thin one that fits.
12mm is rougly 1/2", so you can easily fit a 1/2" torque wrench on the open side of spanner and correctly torque the nut. Spanner-torque wrench should be completely in line.
Also you have do downsize the torque value, since moment arm is bigger with spanner attached.
For this I'm using torque wrench extension calculator from playstore.