UPDATE

It appears that I missed the very obvious solution to this problem (!) which, as pointed out by a couple of commenters, is to use the ‘Trim this field to a maximum length’ option within the settings for the node title field.

I’ll leave this post in place as I think it still serves as a useful guide on how to create template override files for views, but please be advised that for implementing truncation of a views node title it is easier to:

• navigate to the view's 'Edit view viewname' page (/admin/build/views/edit/viewname) and click the ‘Node: Title' link
• check the ‘Trim this field to a maximum length’ checkbox, and adjust the settings as required