Open the sidebar file and put the PHP directly in there, avoiding the need to use one of those plugins?
Thanks for the response Mark.
That would normally be my first workaround, but my current site is using the Thesis theme, which doesn’t have a sidebar.php file, or a header.php even.
I’ve subsequently solved the error, which was a result of the new server having PHP5 set to disable the include function for remote files – a very useful security measure but had me banging my head against the wall for several hours!
This blog post gives an explanation and a couple of workarounds, if anyone else needs help with this issue.