I thought I'd add this here just in case someone had a similar
problem:

Pressing the signature button within Outlook 2007 options didn't cause
the signature editor to appear (also the editor options didn't cause
the editor options to appear)

Fix:

I'd noticed earlier in the day that I couldn't "view source" from
within internet explorer. Deleting the temporary internet files fixed
this, and the problems above.

HTH

Regards,
Mark