Upgrading this site to the latest 6.x release of Drupal seemed to go fine, but I started to get emails warning of cron failures. The specific error being recorded was
Fatal error: Call to undefined function json_decode() in myhostingpath/sites/all/modules/twitter/twitter.lib.php on line 258
Now, although I'd forgotten I'd even got a twitter account or that I'd got a twitter module installed, I assumed that disabling the module in mentioned would just have thrown a similar error for another module. So best to fix the problem.
So in the interests of helping anyone else who's (googled for and) got this problem here's how I've fixed it:
