Yes, I assume that posting something from the UK to Australia back in 1997 would have taken a while - a week or so? But had she have been intending to return, she would have brought it in person.
By the same logic, would Marion not have been looking for a special present for Owen from the UK, maybe something 'cool' from London and have got that in the post in plenty of good time. So, missing his birthday is one thing but then add on an extra, what, at least 7 days postal time she would have accounted for in the UK (if in the UK).
Also, all the shopping and all the luggage. If Marion DID return to Australia for a quick hop over to sort out her finances and RB also headed back, where is all the UK luggage and shopping?