This is now open, as of 4pm yesterday, according to the chap from SWR at the top of the stairs when I ceremonially walked up at 4.29pm.
They only have budget for it to be open 4-8pm weekdays, and he remarked that most trains were too short for it to be much use, as passengers won't walk back on themselves to access it. Nevertheless, I expect when a 12 coach Basingstoke/Guildford arrives on p3 at the same time as an 8 coach Guildford/Woking/Hampton Court on p4, that the down platforms will be much more pleasant places to be.
Skylights are currently still temporary and due to be replaced.