I turned on the debugging I just found. I think I have worked it out ironically after posting..
With the phpbb exclude settings, it is missing file.php. The particular file it stopped fetching was a 514MB file.
Its not stopped again this time.
Though I think adding file.php might have created another problem that it is only indexing about 1,500 topics on my forum when there are around 8,000 topics.
Before I added file.php it got to about 3,478 links. I'm not sure if that is exactly the problem or not.
Is really there should be a option added onto the settings to limit file sizes to 1MB or something. As with this mostly (I assume) being used for site links only then pages crap should not really be massively huge because they should be mostly text anyway.