Thomaso you legend! 😀
I finally fixed it (thanks to your help). That was doing my head in.
I removed all the Leading Overide. Switched to Paragraph Spacing. Then used the Space Before to measure out the gap and it has worked a treat. The final problem with the Sub-header was that I had not checked the "Only at the top of the Page" marker with the "Use Space Before" check-list.
Thanks so much. I owe you a beer!
Upper_Krust got a reaction from thomaso in Affinity Publisher weird Leading issue with headers at the top of a text box