I have now repeated the process of upgrade from Joomla! v1.5.14 to v1.5.15 on my local server - Captcha was OK.
HOWEVER: as soon as I upgraded from Fireboard to Kunena the error re-occurred - ONLY on the forum menu item (all other menu items AOK.
I have posted to googlegroups joomlacaptcha and their man Viktor is investigating. I have reported that the error seems to be connected to the installation of Kunena (or, rather, the upgrade from Fireboard)
Viktor asked me to check the code in the captcha.php plugin file - it seems OK. I have checked that the language file is correct. I suggested that Kupala Captcha liaise with Kunena to investigate.
The googlegroups joomlacaptcha thread can be found here
groups.google.com/group/joomlacaptcha/br...ead/3454e00bc138f844
Thanks for your help.