Can't connect MacBook Air to LED TV anymore

I'm not sure if it was that recent security update, or the fact I've bought a new television, but it seems my Macbook Air no longer wants to play with the television at all.

It's a little old, Late 2010, 1.4gHZ Intel core 2 duo macbook air, and i'm trying to get it to connect via MiniPort/HDMI cable to an LG55LB650V. It was previously connected to a two year old LG 42" television without any problem via the same cable i'm using here (or not using infact)....

So, i'm not sure if it's just the TV doesn't like computers, or if it's something about the software on the Macbook Air? What is weird is the screen flashes in and out once when I plug the miniport cable in, so it knows it's there I guess, but in the system preferences menu it doesn't seem to even acknowledge that there is an external device, the display preferences no longer asks about other displays, or infact I just noticed it also doesn't have that tick box it used to have about showing display preferences in the menu bar? So, i'm wondering if Apple have been tinkering around and this is whats causing the problem. Any help would be greatly appreciated.