Hi Ziggy, sorry for late response!
Thanks very much for your suggestion, but I don't have any code injected into the header (in the Advanced > Code Injection section of Squarespace it is completely empty). This is what is so strange. I have several bits of CSS code in the Design> Custom CSS box but it's specifically only this line that is appearing when the WORK page loads. It's doing the job of keeping the boxes on the Services page a consistent size, but even if I delete this line of code from Custom CSS it still shows up when loading the WORK page. I just can't work it out - it's almost like it's being pulled from somewhere else.
Scratching my head...