My theory, as I said, is very similar to
@MrsEmmaPeel
RB did months of ground work on MB starting with convincing her that he was FNR, the football player who 'almost' met her in 1968. Con-men mirror their victims by encouraging them to talk about themselves in order to insert themselves into the victims story. "Oh, traveling on the Orient Express is my dream too", sort of thing. Marion cannot be compared to JO in this regard, she was far more worldly and took months of grooming to allay her natural skepticism. He also had to include a credible reason for secrecy. I don't think Marion would have believed the "spy" line, MC obviously didn't, but she might have been convinced he was in the midst of a large business deal that required utmost secrecy until completed; perhaps even a deal with the Australian Government. Knowing Marion's close family ties, it would have to be something temporary.
Having completely "love bombed" Marion with a marriage proposal, he tells her the deal is nearing completion, his visa is about to expire and he will have to return to Luxembourg. This forces Marion to make a decision; sell up and go with him or lose him. She decides on the former with every intention of revealing all to her family in a matter of weeks. She was about to marry a millionaire business tycoon and live a life of luxury in Europe.
The name change was his idea but how he convinced Marion to comply and how it formed part of his scam is a mystery. Marion had so much to accomplish in such a short time frame (sell her house, resign from her job, re-organize her entire life) she must have been flustered. He kept up the pressure, he may have to leave early, the deal is almost done etc. so Marion barely had time to question anything. Her new passport is late arriving and her bank accounts still had to be changed.
RB ups the pressure; he has to return early on the 17th June to finalize the deal. He spends the night with her on 16th, she brings forward her resignation. Her passport still hasn't arrived, she had enough cash and money in the Barclays account to tide her over, she barely had time to think so she decided to sort out her bank accounts from the UK. It wasn't a priority to Marion, she was about to marry a millionaire!
RB meets Marion at Heathrow. He tells her it is advisable (criminals target tourists etc) to store her important documents with some of her luggage at Heathrow. He stores his little silver case also and he keeps the receipt. He's booked accommodation at Tunbridge Wells and they spend some time together. This is when he convinces Marion they are essentially 'married', the only thing missing is the piece of paper. Then he has to leave for a few days; he just has a few things to tie up in Luxembourg. He retrieves his luggage and checks Marion's documents and that's when he realizes Marion hasn't transferred her money. He is not happy! He has to change his whole plan. Going back to Australia is the last thing he wants to do.
He advises Marion to retrieve her documents at Heathrow (he posts back the receipt) and go to Barclays in London and sort out the problem. This is likely when she mentioned in her correspondence that she travelled on the tube in London.
Desperate to recoup the time, effort and money he'd already invested, he realizes his only option now is to return to Australia.
The rest is too long to post here.
