Is that a short pruning cut I see at the top (left) of the damaged area?
If so, that could have caused stress for the tissues below it - because the sap flows down the trunk (through the phloem). It is quite a long area of damage, though; ordinarily side-ways flow would limit that sort-of damage to a shorter area under the cut.
Also, if that is a pruning cut, if the tool was infected, it could have introduced a pathogen, which could have spread down the phloem. There are many types of infection a tree can suffer, eg. bacterial canker (I think not in this case).
So I'm leaning towards physical damage or stress, eg. something leaning up against the tree in that area, or sudden exposure to hot sun, perhaps by pruning-away of branches on this or adjacent trees. Large beeches can suffer such sun shock when sunlight floods into a copse following the toppling of one tree. Such sun shock can be a primary cause of stress, and it can render the area susceptible to fungal or bacterial attack - a secondary problem.
I wonder what you see if you peel/prise away some of the damage/dead bark. Staining in the tissues could indicate an infection. Galleries could indicate insect larvae borings (which might be secondary).
Thanks for the information. This probably means it was stress as you suggest as a large neighbouring tree which overshadowed it was removed three years ago and it was also cut back. This has exposed it to much more sunlight.