Oh I've worked it out myself! Right click the folder and choose "synchronise folder". Happy now!
Another solution is when you export, you check the box that says Add to Catalog, in which case the exported photos are automatically added to the catalog, no synchronizing needed.
To provide a "thinking out side the box" alternative answer:
There is little benefit to importing photos you have exported from Lightroom, as the exports could be viewed as temporary, and thus could be ignored; if you ever need the exports again, Lightroom can reproduce the exported file exactly. So the recommendation is to not have exported photos in Lightroom.
Thank you, I will tick the add to catalog button in future (didn't even notice it when I set up my presets!). I have not attempted to re-import photos, The reason I needed to re-export was I needed to check the metadata on my final images and export a lower resolution version of them depending on the lens I used to sent on email :-)