Thanks Richard and Tyler,
Unfortunately, it wasn't as simple as the 'export clipboard' as that has always been unchecked.
I also tried to purge the previews cache, where all previews are cached at 100% (must be a fair bit of buildup for some photographers). Anyway, that wasn't successfully completed as it couldn't find the folder to purge. Which made me remember I'd used CleanMyMac, in a rash mood of cleaning up, the other day. This seems to have changed Bridge to stop updating changes.
When using the CleanMyMac, it did find 10gb of Adobe related files to purge. I guess i should have checked what that was. Doh.
The F5 does work fine, but can take a while with a folder of up to 300 TIFF files.