How should I treat this Oak Tree fungi?

[NewspaperTaxis]

This does recur sometimes, every few years, but it needs treating - obviously I can just get rid of it, but does anyone know how dangerous it is, and if I should paste anything onto the area once it's removed?

I do wonder if it has prompted premature leaf fall during the summer months, but assumed that was because the tree needed trimming or because of the sunny weather.

Just break it off, that should do the trick. You can't get rid of fungus like that, and the tree will cope just fine. The fungus mycelium won't damage live tissue, it will be feeding on dead or decaying matter. Lots of oak trees decay from the inside out, and still manage to live for hundreds of years. Some of them are completely hollow.