Black Screen Audio no picture

Hi having exhausted everything I can try I am feeling frustrated driving me mad......please any help appreciated

I am using a macbook air (used to use a macbook pro for the same function and all worked perfectly) to 1) screen mirror and 2) connect via HDMI (tried both) to my Samsung model Code UE48J5100AKXXU  TV.

In the past via a DNS proxy server or vpn connection I can watch UK TV in SA all channels - since I have got my macbook air I can not 1) get the screen on my laptop to show on the Samsung tv via HDMI - its fine when mirroring but not on HDMI and

2) can not watch channel 4 or ITV I get a black screen with audio but no picture on the TV using mirroring or HDMI but I can watch BBC channels no problem....

Have tried to use Safari and Chrome to see if either work but no luck on either. Its the not being able to watch channel 4 that is frustrating me the most so any suggestions or help (in easy to follow simple terms please) would be much appreciated. Thank you