I am using times new roman bold, within InDesign but only recently a curl has appeared when 'st' are written next to eachother. Does anyone know why this has started happening?

something like this ? https://commons.wikimedia.org/wiki/File:Latin_ligatures_ct_and_st.svg
opentype ligature ?

Yes exactly...is there a way to stop it?

yeah probably. You should search on the InDesign manual.

I have managed to sort it...don't fully understand but it's within the open type section. Thank you!