How to allow second local user access to DISPLAY 0

I have basic desktop machine with two user accounts. I want to allow the second user ( no the one which started X ) to access xserver and run GUI programs. I had this working using xhost from the login user but this no longer seems to work. xhost +SI:localuser:fred now if I do su - fred and try anything which requires X, I get an error to the effect that it cannot access the display. $ xhost xhost: unable to open display "" Please do not reply about how to use vnc, ssh or remote access. That is not the question. Thanks for any help in how to do this, it is stopping me being able to work.