Thunderbolt to HDMI does not detect TV on Yosemite

I am having the same problem as a lot of people with Yosemite and and connecting to a TV/external monitor via thunderbolt to HDMI but have found a sort of a solution or at least a bit more info that might lead to someone having a eureka moment. I have a Mac Book Pro, my partner a Mac Book Air. I bought a thunderbolt to HDMI cable from eBay so my partner could plug her Air into the TV. When the cable arrived, it would not detect our LG TV from either computer. So a bit of playing around and this is what I found.

• I plugged the cable from my MBP direct to the HDMI port on the TV and nothing, no signal detected.
• I also tried from the MBP to the HDMI port on the amplifier of the surround sound system and again nothing.
• However, if I plug the cable from my MBP directly to the TV HDMI and select that corresponding HDMI port on the TV initially still nothing. But if I then plug a normal hdmi-to-hdmi cable from my amplifier to the Mac Pro's HDMI port, suddenly the thunderbolt cable works and the Display Preferences sees the LGTV. I checked that the correct hdmi port was selected on the TV for the direct connection and I could even turn off my amplifier to make sure that I was not seeing the wrong signal and it still worked as long as the hdmi-to-hdmi was still connected at both ends. However, if I then unplug the HDMI cable from MBP or the amp, the thunderbolt port stops working as well and the TV displays "no signal detected".

It seems that there needs to be either a base signal or an earthing through the MacBook Pro's HDMI port for it to switch into an external monitor mode that will detect a thunderbolt to HDMI connection. This is sort of fine for my MBP as I had nothing working before but it seems a bit pointless having the HDMI cable plugged in as well as I might as well just use this in the first place. However, as my partner's Air only has a thunderbolt port and no HDMI port it still doesn't work with her MAC.

Hopefully someone will see this and have a brainwave.