[support] Call to undefined function block_list after upgrade

So, an old drupal site (4.4.x) finally gets upgraded. Everything works OK, through the various database upgrades to 4.7.0, and then I run the update.php for 4.7.3 and it seems to run the last three system updates OK, too.

Then, when I fire up the index page, I get:

Fatal error: Call to undefined function: block_list() in /home/hackneytuc/www/includes/theme.inc on line 935

Why should block_list() be undefined? It's in the block module, isn't it? I've tried switching the status of the block module in the system table directly from within MySQL but that makes no difference - I cannot access any of the site, because this fatal error message comes up every time.

Any suggestions as to what is causing it, or how I can get round it?

Thanks,

Nick