Thanks!
Sorted!
Though not quite as you wrote....
1. In Bridge, Select All for the images to be resized.
2. Select Tools > Photoshop > Image Processor.
3. Tick Resize to Fit box.
4. Put 10000 in the width box.
5. Put 1080 in the height box.
6. Tick Save in Same Location
7. Click Run.
As you rightly said, PS ignored the 10000 figure and resized everything to 1080 in height and the width proportionately.
Many thanks!
D.