I have raised this in the WordPress forum because I assume it may have a general answer when an installation screws up the system.
I uploaded the bbPress folder as instructed for a first install.
Got “Fatal error”. So I deleted the folder and re-uploaded it. I have now done this three times on two separate days. I have meanwhile also uploaded a Log in form widget which is happily working.

Yet again, after uploading after third attempt:
Fatal error: Allowed memory size of 41943040 bytes exhausted (tried to allocate 30720 bytes) in /home/aconbury/public_html/wp-admin/includes/class-wp-list-table.php on line 304

Earlier, second attempt I got:
Fatal error: Allowed memory size of 41943040 bytes exhausted (tried to allocate 30720 bytes) in /home/aconbury/public_html/wp-admin/includes/class-wp-list-table.php on line 866

Only way back into wp-admin is to delete bbPress YET again! Would welcome any ideas.
Bog standard set up, host thirty odd sites but never a WordPress one!

Latest version of wordpress (three days old) and yesterdays bbPress)

[@johnjamesjacoby Keymaster 11 years, 6 months ago]

Your server is running out of memory. You’ll want to increase the amount available to PHP. Searching the web for the preferred way of doing this with your webhost is probably best.

[@jnorris235 Participant 11 years, 6 months ago]

I was unaware that they limited memory in this way.
Thank you.
Now resolved, thanks to your help.