I have airplay button but it won't work from imac to ATV. Phones are a-go however...

Question

Current household setup:
-most recent imac that didn't get lyposuction. Not upgraded to 10.7.5. Currently sitting at OS update one before that.
-2x Iphone 4S
-Apple TV, most recent model, updated to latest update.
-Linksys 2600E wireless router
So currently, I can airplay and mirror from both iphones to the Apple TV with zero issues. Music streaming, youtube mirroring and browsing all works flawlessly. However, when I attempt to stream a .mp4 movie from itunes to the apple TV by either playing it on the imac OR playing it from the Apple TV it fails.
Everything worked perfectly until I updated the Apple TV to 5.1. The moment that update went through, it "broke" airplay between the imac and ATV. Now, in iTunes I still have the airplay icon and it's clickable to select Apple TV for streaming, but when it begins, it just sort of hangs then eventually times out.
I've managed to get it to work briefly (for 20 minutes) by changing some router settings, but nothing permanent.  From memory (I'm currently at work) the settings on my router are as follows:
-WPA, non hidden SSID
-MAC Address filtering OFF
-2.4GHZ on 20hz only (as suggested by apple support page)
-5ghz on 20 to 40
-Port forwarded the following ports, now this is from memory so may not be 100% - 123, 3689, 5353  http://http://support.apple.com/kb/HT2463
-the band or channel signal is set to Auto for both.
These are the settings as recommended by Apple and it still doesn't work. I think that the 5.1+ update screwed with some wifi settings in the ATV airplay fuctionality but I'm pretty confused about everything. How come it works via iphones, but not from the imac? (which is also on wifi)
My iphone, iMac and Apple TV have the same home share account logged in, all on the same WIFI network, through the same router. It's odd.
If anyone has suggestions or answers, I'd be thrilled to try them out or look into them further.
Regards;
-T3sk0

Answer 1

You would have to have a credit card with a valid billing address in the U.K.