So if I understand that right, you have partly code for 1.5.1 and partly code for 1.5.5 on your site (or else the file in question would not be called).
You can either remove the latest westminster_new file set and then install an older file set of this template which is only compatible with ZC 1.5.1 (but not higher) OR you bite the bullet and do a full ZC upgrade 1.5.1 > 1.5.5.
Personally I would choose the latter as ZC 1.5.1 is well outdated.
Bookmarks