If that's oozing out of the tree trunk, specially if it has a smell, and continues after you've tried to flush it off, it may be bacterial canker, which is not good, means you need to get rid of it. Could also be slime mould though...

To me it looks suspiciously like borers. Do you have those in the UK?

No, Tugbrethil - we have various little beetles which get under the bark in very old trees, but not borers.

Count yourself lucky! Means that I really can't help with the cause of the oozing, though.

I do count myself lucky, Bamboo - we pay a price with the poor weather, but the advantage is far fewer nasty beasties....