I've spent a little while trying to work out why my email signatures weren't working, until I figured out there's a global setting in Email Setup, which I was overlooking due to the way it's displayed, and a lack of documentation (once found it's self evident).
The setting for email signatures is grouped below the Bulk Email Settings, which is a little confusing, since it's more of a global setting. I was thinking it was part of the bulk email settings (email broadcasts) rather than global behaviour and overlooking it as a result. This was an oversight on my part.
May I suggest a separate heading or grouping under a different heading to improve that UI?
Email signatures are fully functional now I've found the setting.