Version 2.1 of ECaptcha Spambot Protection is now available
Double Doh ! Found mb_convert_encoding is part of a mbstring library (http://php.net/mbstring) which is not installed on some webservers. An alternative is now used if its not available.
Doh, found mb_convert_encoding does not convert html characters, which could muck up the ecaptcha form. Adding htmlspecialchars alongside fixed the problem.
Fixed is the display text for foreign characters, a limitation of htmlentities, switched to mb_convert_encoding instead which seems to do a much better job.
Thanks goes to David Coll (http://arcur.asso.fr) for pointing this out.