@kiwidude @jackie_w - thanks for all that - very helpful. You're absolutely right, jackie. I missed the difference in how the code is inserted somehow and now feel a bit stupid for doing so.
The colour and font-size changes, incidentally, were largely temporary, to assist with my amateur trial and error methods.
However, I still don't understand this: some months ago, I went through my entire library, using Modify epub exclusively to replace jackets - a blanket approach which replaced ALL my jackets using the modified template/css. The jackets done at that time have the code in the jacket.xhtml file, and read perfectly everywhere. I will have to print out the code and do a side-by-side comparison to see what and where things differ in the jacket.xhtml files.
Thanks again!
@jackie_w The main reason I insert jackets is to make the series info available to Mantano, which will sort on series - presumably that info comes from the jacket?? I haven't had any problems at all with Mantano since they fixed the fairly minor issue of incorrect cover display on the Nexus after me nagging them. : (They have a great responsive attitude in their support team, BTW.) I do very basic reading, with only the very occasional note, bookmark etc.. The other contenders are dreadful.