Bosch Exxcel 1400 Error F04 - can't open access hatch?

I have followed advice in previous postings but cannot get into the filter becasue the ciruclar access won't open. It has a vertical bar which you have to grip and turn - it is very stiff and feels 'gritty' I can only move it 1/6th of a turn and then it sticks. If I try to push it further, the plastic seems about to crack. Does anybody know how far it should turn before you can open it. Also will WD40 help (or will it damage the plastic)?
Thanks