I would like to be able to create bold within the webform itslef (rather than the xls form). I tried using excel and wrapping the text in _ _ but this did not appear to work.
What exactly do you want to do, please? Can you show the wanted result?
The search function of this forum may help you; e.g. Search results for 'label bold enketo' - KoboToolbox Community Forum and esp. Question labels - entering spaces and bold text.
You may also take a look at the documentation, please:
Enketo-powered web forms only support Markdown styling. Styling is not yet supported in web form choice labels.
Form Styling - ODK Docs (Markdown)