Mini Display Port to TV problem

Hi, I bought a mini display port to HDMI adapter for my late 2009 iMac.

Displays in Settings shows the TV, with the correct resolution, but the TV shows no output from the iMac. I've checked the TV with a PC (OK) and I've done Control-F1 but still no response on the telly. From the iMac end it looks like there's a second monitor...

Anyone seen this before?

thanks

Phil