The received wisdom used to be that since HDMI is a digital signal, any old cable would do so don't waste your money on flashy stuff. In fact it's incredibly prone to problems from connections, EM interference, poor quality internal wire causing digital errors, sharp bends round walls, etc. Some older monitors and TVs emphasise this. 

Is there a staff kitchen next to the boardroom with a fridge/fan/water-cooler/whatever in it? (Especially if it wasn't there before). Is there a huge run of Ethernet/mains/sound cabling or similar sharing the underfloor route?

How old is the TV/Monitor? Our old plasma telly distorted and complained like mad unless it had decent shielded cable plugged into it.

I still don't completely understand your setup. have you been able to remove the chased-in cable to test that separately, i.e. have you been able to test it when it's free of its conduit with or without the female/female extension? Might be worth spending a couple of pounds on some clip on ferrite chokes if all else fails.

The chased in cable going to the floorbox from the TV works fine - I've plugged a laptop directly into that and all's well, as is the rest of the cabling when removed from the floor box (the female/female cable and the lead from that to the laptop). It's just when the female/female is used in the floorbox that we see issues. It could well be some EM interference from adjacent power cables, but the direct connection to the TV in the floorbox inamongst all that works fine. Apart from the mains cables for the room itself, there aren't any others running through that part. Most are in the ceiling void anyway. I don't know if the old cables are shielded or not, and it's almost certain that the female/female connector isn't, but I can test that this aft in a similar setup in our main office boardroom.

We don't get the cheap cables, we go for the lower end of the branded ones.

The TV's an 5-6yr old plasma. Did you see a similar kind of distortion to that which I'm seeing?

No, just periodic break ups across the whole picture. 
 

I don't know if it makes any difference, but the dots and other artefacts appear in the same place each time something's connected.

so, the female-to-female adapter/cables are shit even fresh out of the box? I don't currently see what else it could be. Possibly, messing with resolution (to a lower one or a native one) and/or refresh rate would work around it. I had a cable that could only do 720p once, that was weird.

Seems that way. :C

I'm over there again early next week, so I'll get to mess around with it again then.

What milko said, sounds like the coupler is rubbish.

I've had HDMI cables that don't play nice with CEC communication (controlling TV, amplifier, volume etc). Also recently had a setup where CEC was working great, then just decided it didn't fancy it anymore. Changed all cables, changed all settings, couldn't get the magic combination of whatever along with a suitable sacrifice to get them talking properly again. Total ballache.

It's more a case of if everything works with a cheap cable, a higher quality cable will do jack shit for improving the image quality any further. This is despite some people with £100 platinum-plated HDMI cables adamantly claiming to see "richer colours" or other such crap with their more expensive leads.

However where a cheap cable/adapter combined with kinks, bends and other interference results in the signal breaking up (as is happening here), a more expensive and better shielded cable may be worth trying.

I guess it's more down to consistent and suitable build quality, along with shielding, rather than bullshit coatings and oxygen free bollocks.

There still is a market for people who have insane beliefs about the benefits of graphite coatings on oxygen-free copper inside medical grade silicone insulation. In the audio world, it reached its peak in the late 90s with a gent called Russ Andrews who marketed paper dots that you could peel off and stick to things like fridges, water tanks, televisions, etc. These had been 'treated' with his proprietary purified water and would make your hifi sound better. He's still in business banging out his special plug fuses and so on.

If the coupler (that's so much easier to type than female to female) is rubbish though, why would it work in a different office. The coupler seems the weak link at the office with the floorbox, plugging straight into the TV there from the laptop works ok.

The coupler works fine at our main office though. It only fouls up in the other office, which points the finger either at the environment there, or a combination of things in that setup that I don't yet understand.  :-/

If I get time this afternoon, I'll set something up in the server room and have the coupler very close to the back of the server cab.

Could be a larger amount of interference in the other office. I haven't read back but is the connection box next to or does it contain a load of other electrical junctions that might lose their shielding in that area?

Wrap it all in lead.

or,  

I like the wrap it all in lead option. It might stop people tinkering with it.

I tried the coupler in a small scale setup with a tft monitor in our server room, and have literally had it on top of the four 3 phase commando connectors in there and inamongst a lot of other power leads and everything was fine.

I'll cancel the lead order. :C

I had a look at his site earlier on and the Kimber brand they sell rings a bell as a high priced and even higher bollocks name from when I first started looking into hifi stuff.

There's this on there. Crazy stuff. There's even an option for getting your cable frozen. I suspect the chill might not last once I've got it installed and cranked the heating up.

Wow! That's just ridiculous. I'm almost tempted by the £1,195 1m USB cable. It comes with its own carry case :O And definitely the cryo treatment. Although no option for burn in with the USB cables :(

Ok, er, they're suggesting adding an earth rod with their RF Router to dump additional ground noise from the system. To me that would be worse cos the system would have two different ground references.

Hah, just had a look at that. I love the idea that it comes with a carry case. I appreciate cables are portable, but once it's in your hifi, does it need to be? Handmade? For that price I'd want some machine precision engineering, not some dude with a soldering iron putting my cable together.

Possible ground loop right there. Stupid idea!

Well, I think I got to the bottom of it. The cable coming from the TV into the floorbox is missing a pin at the floorbox end. It's marked up as Channel 0+ (pin 7) on this diagram:

https://s.hswstatic.com/gif/hdmi-connector-diagram.gif

I guess plugging a device straight into it gets around whatever the problem is (grounding maybe), but using the extensions just exacerbates it.