Really poor WiFi Signals.

Hi all. Really hope someone can help me here.
I am using an Asus RT-N66U. Signal for wifi at home has never been great, but on a visit for Christmas to the in laws, I was stunned. My father in law has the exact same router. His house is 2 floors and god knows how many rooms. The router is located in the study on the ground floor.
There wasn't a place in the house that didn't get me full signal.
So when I got home I was annoyed my router is hopeless.

To better understand I've made some images. First, this is the WiFi map of my apartment. It's a 2 bed flat on one floor.


As you can see, I shouldn't get that much drop off for such a short distance. The wifi in the toilet/bathroom is unusable and the 2nd bedroom is hopeless.
When I pop outside the front door, the signal also drops. Not to the point of unusable, but watching youtube is futile. I'm on a 125meg connection by the way.

Here is some wifi scans of the area. Firstly, right beside my router.



Mine is the red one, with the silly name. Don't ask, my partner did it.

Now another scan taken from the bathroom.



Finally, my settings for the router:




The things I have tried:

Changing wifi channels. Curently now set to 13u
Increasing the TX power to 100.
Moving router away from DECT phones and the TV area. But due to cable restrictions, I can't move it any more.
Installing custom firmware on the device in an effort to increase range.
Used 2 other wireless routers, results similar.

The only thing I can think of is that there are too many other wifi signals in the area. But if that is the case, then wifi is more useless than I thought.

I've also included some other images to help. First image is of a WiFi scan (with a new SSID) from my living room and one from the bathroom.



I'm pulling my hair out over this. Unless my walls are lead lined, I can't fathom it.


Thanks in advance!