I tried to find a long 18mm spanner today without success, but did buy a six sided 18mm socket.
I had a go with it on the long breaker bar and had a small moment of success this evening, although i nearly put my shoulder out in the process
This time, without the socket slipping I managed to tighten the stubborn bolt about 1/12 of a turn, fortunately i then undid it the same amount. I did this twice and then gave up as it was getting too dark.
I gave it yet another soaking with penetrating oil, ready for tomorrow's attempt.