OK, got a bit further. The same TT2 section was in httpd.conf and apache2.conf. Now only in apache2.conf and starts OK. If I misspell one of the TT2 lines it fails, so def. being called.

Apache log now shows

 [error] file error - header.tt2: not found /var/www/html/index.tt2
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/var/www/html/index.tt2 contains

[% INCLUDE header.tt2 %]
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Presumably that means header.tt2 which does exist in  TT2IncludePath /avbase/include/tt2/
can't be found. Why I can't explain!