Hi Visanth
I'm happy to say that my moodle is properly working now. I had 2 problems:
1. incorrect setup of php-fpm
The change to apache 2.4 was not the problem in itself but I had set up php-fpm incorrectly and it was failing to allow file downloads. I've simply reverted to using the old setup of php5 with mpm_prefork instead of php-fpm with mod_proxy and mpm_event for now. This means I've lost a hoped-for speed gain but am no worse off than before my upgrade.
2. incomplete restore of my moodledata backup
I'd copied my backed up moodledata across the network a couple of times and one of those operations wasn't complete properly. I could have saved myself a lot of time with du -sh moodledata (which shows total disk usage of the directory moodledata). My original backup was 88 GB but the restored moodledata directory was only about 30GB.
Your suggestion about slasharguments looks helpful to me - checking the apache documentation http://httpd.apache.org/docs/current/mod/core.html I wonder if my setup with php-fpm may have defaulted to AcceptPathInfo off. I'll do some more checking on that.
Thanks for your time. It's really helpful to have someone to talk to when things go wrong!
Carol