Thanks everyone for the comments and ideas.

I will seriously consider using a walking stick in future. I don't believe pepper spray is legal here but I shall investigate further. I usually have a folder on me, but I can't see how it would have been of any use in this situation.

In hindsight, I should definitely have let go of my dog before the other dog lunged, he is perfectly able to take care of himself, was older and therefore had a psychological advantage and has a very thick coat to protect him.

I'm still shocked at how little time I had to let go and at how painless the initial bite was. The first I knew about it was when the blood started dripping. I shall certainly be more proactive in future situations.

Andy