@Nick_1964,
Have you considered installing the custom firmware,
PRSPlus, on your Sonys (not the T1)? This will let you store a set of css files on your Sony and pick the one which suits any particular epub best. So you could store a selection with different custom fonts or different values for font-size.
Depending on the particular epub you may not need to edit it at all.
[Edit: Sorry I just referred back to your earlier post, I see you are already using it.