One of the things that the film Mary Poppins is most remembered for these days is the dreadful job Dick Van Dyke does with a London accent: whenever he opens his mouth in that film it's as if that's where vowels go to die a horrible death. But back in 1964, Mary Poppins was a very big deal for Walt Disney.

Not only did it win an impressive five Oscars, it was nominated for thirteen in total. It was also a big hit with audiences and, therefore the box office, with its eventual takings hitting well over the hundred million dollars mark. No need for a spoonful of sugar there.

All this would have been music to Walt's ears, especially when you consider how difficult it was to get Mary Poppins off the ground in the first place.

When Walt Disney's (Tom Hanks) children were growing up, they constantly told him how wonderful the book Mary Poppins was. He agreed and loved it so much that he wanted to make a film version. So he wrote a letter to its author, Mrs P.L Travers (Emma Thompson), asking for the rights. She turned him down.

He then wrote to her the following year. She turned him down. And then the year after and the year after that. Undeterred, Disney continued asking for over eighteen years until Travers finally succumbed and said yes.

This would have been a time to rejoice, but it came with a hefty caveat – she would have final approval on everything, otherwise the rights would remain hers. Amazingly, Disney accepted her terms. He then flew her over to Hollywood; if he thought that the last eighteen years had been tough, nothing could prepare him or his creative team for the incredibly hard, uphill struggle ahead of them with the belligerent Travers at the peak of it.

I'm not sure where the rumour that Jimmy Saville was here came from, but it's killing business.

As this film comes from the House of Mouse, you were only ever going to get one side of Walt Disney: namely, the caring, charming, productive genius behind all things Disney. As you would expect, Hanks can pull this kind of role off in his sleep. He makes it look all so effortless, but that shouldn't belie his performance.

He couldn't do what he does without Thompson acting as the perfect foil; her Travers is a no-nonsense kind of woman who not only expects things done a certain way – her own – but she also expects all those around her to conform to her expectations. Again, Thompson has had so much practice with characters with this kind of make-up that it could hardly have been a stretch for her. Still, the scenes in which the pair are together are easily the best.

Unfortunately, a secondary story isn't so much weaved into proceedings so much as brutally crowbarred in. It concerns the relationship Travers had with her father Robert (Colin Farrell) back in her native Australia as a child. It would have worked as one brief flashback, but sadly director John Lee Hancock (who also directed the insipid Oscar winner The Blind Side) elected to use far too many of these pointless flashbacks as a crutch. In doing so it doesn't just break the rhythm created by Hanks and Thompson, it almost brings proceedings to a standstill. Hancock should give the audience a little more credit: we get it, her dad screwed her up – move on already!

These scenes don't just ruin the relationship of the central characters, they positively take the shine off the whole film. Paul Giammati's Driving Miss Travers sub-story is incredibly sweet, and the scenes that feature Jason Schwartzman, B.J. Novak and Bradley Whitford as the Disney creative posse bring a nice line of gentle humour to proceedings - but all this is slightly ruined by far too many Outback flashbacks. It's a surprise and disappointment that given the talent at his disposal and a compelling enough story, Hancock chose to devote so much of the film to a dull and irritating non-plot.

Saving Mr Banks had real potential, possibly even worthy of a golden statue or two like Mary Poppins itself. But having lost its focus, it has been prevented from being truly supercalifragilisticexpialidocious.