Doesn't matter, I've found a workaround.

In case anyone else has the same problem, The Form worked perfectly once the
it was closed and re-opened manually from inside Excel, but the Intranet
version did not understand the automation Workbook Open on first load, and
upon losing focus to another application it was trying to re-open the
Workbook and reload a second version of the Form over the first one, which
was causing it to hang.

I couldn't leave it up to users to close and reopen the form, so instead of
automatically loading, I've set a Command Button on the Worksheet to manually
load the form, which gets around the problem but I still have no idea why
this should happen.