I was just wondering if anyone has had any issues with the rear window demister and how if its easily fixed. The windows were a little moist this morning and usually by the time I get to work the window is clear. But not this day! I'm not sure how long its been like that but is it an mot failure? Its booked in tomorrow.

Not an MOT failure.  Start by checking you have +12v at demister.  If not consider switch, wiring, relays & fuse, and also earthing point.

Well I haven't checked for 12v yet but the light on the switch comes on when the buttons pressed. So I think the fuse is OK. Just another quick question though, on my previous car, a mk2 Mondeo, the demister stayed on for a set time, maybe 15 minutes then switched itself off. But the octavias stays on permantly. Is this normal?

mines always showing the orange light when pushed in whether oh not it stops at some certain temperature and come back on i dont know i just turn it off once cleared.