Not sure if its a setting I may have changed by accident or something to do with the light sensors?

It's very likely that you changed a setting accidentally, or that the light changed and you didn't change any settings to compensate. A passing cloud can make a big difference in the amount of available light. Also, if you were shooting in the late afternoon, the light can fade very quickly.

Also, the metering mode you use can have a large effect on what the light meter tells you about the "right" exposure. The part of the scene that's used for metering is determined by the metering mode. For example, Spot mode looks only at a tiny part of the scene, while Evaluative mode looks at nearly the entire scene. So, if you had the camera set to Spot mode and you pointed it at a bright object, like a light colored shirt, the meter would read a lot of light and you, in response, would adjust the exposure accordingly. But if the rest of the scene is significantly darker than that shirt you'll get an image where the shirt is exposed right but the rest of the scene might be too dark. If you want an overall reading for the scene, use the Evaluative mode.

Do I need take my camera for repair?

Probably not. Try setting it back to an automatic mode, like P, Tv, or Av, and see if photos taken when the camera is adjusting the exposure are better. If that solves the problem, you just need to pay more attention to the light meter and metering mode.

Note that when I shoot in AV mode and auto ISO I get the correct exposure at f/8, 1/13, ISO 6400. (Seems like a rather slow shutter speed and extremely high ISO for broad daylight pics!)

It sounds like you might have the ISO set to A or automatic, and that may be part of what's causing problems for you. Auto ISO can be handy, but while you're trying to get the hang of manual shooting you don't need the camera second guessing your exposure choices. Pick an ISO setting that gives you a reasonable range of shutter speeds and aperture settings and stick with it for now. If you're shooting outside, 100 or 200 should be fine.

As I'm sure you know, Av is "aperture priority" mode. That is, you set the aperture and the camera will then adjust shutter speed and ISO to produce a good exposure. So, if you're shooting at f/8, 1/80s, and ISO 100, but the camera thinks you need 1/13s and ISO 6400 at f/8, then your manual settings provide about 8 stops less light than the camera's choice. That's a huge disparity, and it's no surprise that your shots are underexposed. Each stop represents a doubling or halving in the amount of light, so a difference of 8 stops is about 28 = 256 times as much light.

Read up on the various metering modes that your camera provides (the owners manual is a good place to start). The key to getting the hang of shooting in manual mode is to keep at it: shoot more, experiment, and keep looking at how your exposure choices affect the final images.

Update: Given your sample images, your camera is fine. You've got one properly exposed shot in Av mode, and one dark one where you've chosen settings that give you 4 stops less light than the other. It's no surprise, then, that your image in manual mode is a lot darker.