Can I install DokuWiki straight onto SBS 2008?
Without a webserver it wont work.
All the searches I've made show people have IIS on their SBS 2008. Is this coincidence?
No, installing MS Software on MS software is the easiest part.
Do I need to install IIS? I'm sort of assuming DokuWiki needs some sort of web platform and hence the need for IIS.
Yup
I assume I also need IIS to instal PHP on? Or can I install that straight onto SBS 2008?
You can install PHP with the package iNstaller supplied by MS. Php is a scripting language interpreter with generates html pages defined in scripts (DW), iis sends these pages to the enduser.
Can someone please clarify this? If I do need IIS and don't want to install IIS on SBS then what are my alternatives?
Well you can use apache or nginx or another alternative.
Conclusion : use the MS packages and you are up and running in short time. The others have special needs and will take much more time.
As a tip : consider an external hosted webservice , it can start very cheap , and you have your DW up and running in no time.