Apple tree disease?

Hello,

Can anyone help me identify what is happening to my apple tree trunk and what do I do, to keep it alive? I can google, but I don't have an eye for a problem that an experienced person has.

It looks like
1 a hole under the bark,
2 some areas of the bark are gone, others have strips of bark over the top.
3 A bruise
4 it's about a 5 inch across circle
5 Didn't look like any insects were involved, but I only had time to go, EEEK this weeks GQT, photograph.
6 rest of tree looks unaffected but
7 it's in the middle of the horizontal part of the tree-it fell over as a sapling and had to straighten out.
8 I thought that picture was closer than it was 
9 The fruit and leaves aren't blemished in any way
10 Could it be apple canker?
11 Do I take the protective cover off to see if it's hiding anything?

The echo between this weeks Gardeners Question Time bit about the apple tree (1 minute 40) and "our" tree is haunting me. Someone's previously very productive apple was pruned by a neighbour and the panel said it wasn't is pruning, it was this dark patch on the trunk. They suggested scab so I am looking into that.

Please help save "my" tree.

Modify, been googling how to tread. it maybe canker (any agreements?) Splash back, that's how it spreads, no scalding it. I'm worried since it's on the top side of a horizontal portion of the trunk. That's like removing an ab on someone doing sit ups. Maybe better that than this disembowelling it.

I'm watching Huw's youtube podcast but the post won't allow me to copy and paste, it wasnts to have the word "list" in there. I googled How to Identify and Remove Canker from Apple Trees.

We think the tie probably caused a wound that allowed the canker fungus in.
While it was the cause, enough of the trunk has been eaten away that it is loose and won't cause further damage. Since there's no bulge above or below it it's not been so tight to cause pressure malformation. Taking it off immediately was more dangerous than leaving it there. The tree's diseased, probably canker fungus which is spread by splash from water or touch.

Cutting the tie would contaminate the loppers which would be spread to other tools or plants. We have a second apple and a pear (also susceptible) within 2 meters and an apple tree in the bottom plot. All which need pruned in October, which means the loppers would be a reinfection route. We're not risking that for the sake of a day or two when the weather's due to be dry and we can get everything we need to tackle it. In addition at least 4 other plots on the site have apple trees whom we feel we have to warn. All of which could be infected. Or could be infected or by us if we do this wrong.

We have to check every branch of all 4 trees for this infection and remove everything suspect.
When people treat canker they have to cut it out back to healthy wood and remove every single piece of infected wood then burn it on site. We can't take the offcuts home as there is an apple tree nearby. It's tree gangrene basically, so it won't heal up. Had it been a small branch then yes, we'd crack out the loppers and have it off, but we'd need to wait until we could sterilise and burn. But it's the trunk, it affects one whole side of the trunk quite deep and could easily kill the tree within a year or two.

We have to act carefully or it's stuffed. We didn't have a pruning knife with us, we don't have heal and seal (ordered), a towel or sheet to catch the off cuts and anything to burn the off cuts with. Acting rashly would have done more damage and possibly infected the other trees.
We have time since the fungus won't fruit until later in the season. We're lucky, it's been dry, it's due to stay that way and I haven't fed or watered it. So it's staying put whether it likes it or not.