Well, having spent the best part of half a day trying to find a problem with an Excel Add-in not working on a deployed machine (it worked just fine on the development machine), it wasn’t appearing in Excel. Not even an error message. Finally though, I’ve found the answer.
Having trolled around a load of websites, and learnt a load about install kits and how to install an add in into Office applications, I found an environment variable.
Set VSTO_SUPPRESSDISPLAYALERTS to 0
It then gave me an error message, saying it didn’t have an assembly. Easy to fix when I got the error message.