paper size on sheet tab showing approx x50 too large

i have the model space drawing set up in mm at 1:1. When i go to the sheet file and import a paper size i have set up to A3, 420 x 297mm, and activate, this sets the paper background approx. 50 times too large. This has only happened recently, it was previously working fine. Have i changed a setting by mistake that would affect this? thanks

(MAC version)