Hi mhaffie,
Thanks very much for your help. I have trawled through forums and visa websites and find conflicting info (some quite old!) - the good thing is, different approaches seem to work for different people, so that's a good start if you ask me! We've now got 3 Australian citizens completing form 888 in Australia, however our closer friends are here in the UK (those who have seen our relationship develop more recently). So I think a mixture of official signed and witnessed 888s from Aus, and a few statements signed by a nurse, for example, from the UK will hopefully be sufficient, based on what I have read.
Good luck with your application if you're still waiting on a decision