hmm I should be doing around 160kg, my back is weak!
There's no 'should'.
Bear in mind these are 1 Rep Max/Singes. I deadlift usually between 1.6 and 2.0 times bodyweight for sets of between 4-8. However I consider myself to be reasonably strong at this exercise.
It's not a pulling exercise, push through your heels. It should be felt in your hamstrings and upper back mainly. If you feel your lower back getting 'tight' during the exercise or pulling, you're rounding and doing it wrong.