Question
Have a common Ash which is for all intense and perposes pretty much alive, but it's trunk is very narled and corky.
At the bottom, you can put your arm about a foot up it from the front, how much more I don't know as I can't reach.
It has been like this for at least six years to my knowledge and at one stage had Honey fungus growing at the opening where as mentioned the hole is.
It always puts on leaves and the samaras always appear every year.
I wonder if a strong wind may bring it down.
It's about 30-40 foot high. What do you think has happened? Could it be mechanical damage by an insect that has let in a pathogen and the callused bark the healing process?

Answer
The orginal wound could have been caused by any number of things--I would guess mechanical damage. The damage broke through the cambium layer (living cell of a tree trunk) and allowed  a fungus to enter the woody part of the tree trunk. The fungus is slowly "eating" away the wood inside the tree. This is a very slow process and the speed depends on the moisture level and the health of the tree.
From what you discribe the tree seems healthy (full leaf crown and flowering). The callused bark around the wound is where the tree is trying to out grow the fungus damage (called Compartmentalization). There is always a "chance" that any tree healthy or not could be blown down by Strong winds. I would not be concerned about this tree. I would clean any loose wood and rotten wood from the opening and fertilize the tree and keep it healthy and let the tree try to take care of the wound. You do not want to scrape the rotten wood out but pull out what you can reach with your hands. Some people use to recommend filling the cavity with cement etc. but unless this is done properly you will trap moisture behind the fill and cause much more damage. Just keep the tree healthy and it should be around a long time.