Yes Stephen and thanks. It was that. Strangely, I had to first set it back to default permlinks and save it and then set it to ‘Month/Name’ and save it again so it re-writes it back to the htaccess file to get it to work. Originally I had permlinks set to ‘Month/Name’ but obviously BBPress doesn’t pick this up on activation it seems and thus resetting permanent links is required. Interesting. Thanks.
I’ve just upgraded wordpress to 3.8 and the problem persists. Other than the bbPress plugin I only have 2 others in my install: wp-no-category-base and google-sitemap-generator. I have tried disabling these and it makes no difference.