TonySupport… thank you for responding.
Yes, I have an actual page ovni-users.org.uk/register/ and I had set it as the register page in BuddyPress settings. I am not sure why putting the bbpress forum short code on it would help, but I have tried it with no effect. I also tried putting the [bbp-register] code on it and no effect.
I have now tried setting the BuddyPress register page to none but leaving the [bbp-register] on the page. In this case I get a minimal register form without the BuddyPress fields but on a full width page with no sidebar as specified on my Attitude page settings. So in this case, the no sidebar option for this page is working. [But this is not a solution as I need the BuddyPress extended profile login stuff.]
With my page set as the BuddyPress register page, I have tried deactivating all plugins except BuddyPress and the register page has the sidebar still, so it is not a case of some other plugin causing the problem.
I conclude that BuddyPress is generating its register page in a way that overrides the Attitude page layout selection for that page.
Any further advice will be most welcome.