Reply to Neil: If you achieve a nice tight seal then the woodlice would be kept away, and this is the aim. Before filling with concrete, scrape out all the loose, rotten wood you can. Once dry, fill with concrete, just as you would if completing a DIY job on a wall in your home.
The combination of a hole in your tree and rotting wood does mean that your tree isn't in the best of health, but this treatment should prolong the life of your tree.