The Diana film shows a lack of respect for her story

This film is let down not by its struggling star or the forgettable supporting cast, but by the storyline which meanders through the last two years of Diana's life

German director Oliver Hirschbiegel won worldwide acclaim for his bunker-based Hitler-flick, Downfall...now he has directed a clanger that does for Naomi Watts's career what the Luftwaffe did for Coventry...and Diana the movie may well be the downfall of them both.

This film is, as many other more worthy critics have already pointed out, as wet and flat as Norfolk. I haven't felt so cheated of £11.50 since I bought that lemonade in Dublin.

We open with a shot of Diana from behind, walking through the rooms of her glittering Parisian hotel suite. These, we know, are her final moments. The intention is to establish a poignant and ominous tone from the off. The score hums at us and the dimly-lit, cavernous rooms make the figure, wandering to the marble sink and then to the bedroom, seem lost and lonely. Her trousers are too baggy. That is my main thought. Diana should have gone to Gap.

And then she turns and we see her. Naomi Watts, revealed as the late Princess of Wales and Queen of our hearts. I know this respectively. In truth, I spent the first half hour of the film thinking: “Wow, Clare Balding's lived a life!” Admittedly, the make-up department had a terrible time of it. Filming had to be stopped half-way through when Nicole Kidman sued Watts for stealing her face and Keith Lemon demanded his hair back.

Naomi Watt's portrayal of Diana is pitiful, contrived and pathetic. Helen Mirren did a beautiful job portraying the Queen in a way that appealed to us. Watts, on the other hand, offers us something far less charming. Her Diana is...silly. A more complex description would be unfitting for her performance. I hear she prepared for the part by watching endless hours of archive video footage. Unfortunately, someone switched the Diana tapes for a Little Britain box set, so we are left with a strange mix of Marjorie Dawes and Emily, the bloke in a dress. If you think I'm being unfair, watch David Walliams giggling behind his hand and then go to see this movie.

Ultimately however, this film is let down not by its struggling star or the forgettable supporting cast, but by the storyline which meanders ever-so slowly through the last two years of Diana's life. I suppose the writers didn't have much to work with. Only the tale of a future Queen and most glamorous woman in the world becoming embroiled in a torrid mess of betrayal, intrigue, deception, doomed love and tragic demise. Surely the drama was unavoidable! Not so it seems. Avoid it the writers did. This is a bland love story between two seemingly bland people. It is a plotline from a soap, and a lazy one at that.

The script is awful. Okay, so it was written by a teenage girl as part of the Duke of Edinburgh Awards Scheme, but still. Many of the most horrific lines are given to the character of Dr Hasnat Khan, the surgeon Diana falls in love with after her split with Charles. 'You don't perform the operation, the operation performs you,' he says over dinner. A very witty line, that one. What surgeon hasn't been bossed around by an appendix? And then we have a gloriously chucklish moment when Diana decides to cook him some dinner and walks towards the kitchen. 'Pretty hot stuff eh?' says Khan. Diana turns back. 'What?' She asks, shocked. Dr Khan panics, realising how easily a completely normal and utterly natural comment can become a ridiculously clumsy shoe-horned double-entendre. 'You in the kitchen,' he explains, wincing visibly.

That's not to say Naomi Watts doesn't get to deliver her own bangers. You might imagine the scripting of Diana's landmine-dodging trip to Angola would be afforded subtle and careful treatment. Not so. Here's Diana, worrying about the press coverage at home: 'Did they publish the pictures of the kids with all their arms and legs blown off?'

We are thankful the real Diana avoided the mines but Watts trod right on a whopper there and I don't mean Burger King. BOOM. Did she read the script?

A couple of moments in the film do win through. I felt a sincere pang of sadness as I watched Diana kiss her boys goodbye and wave them off for the last time. Harry turns back to look at his mum and smiles. The last look. And there's a fleeting moment in a nighttime break-up scene in an unidentified park when Watts remembers how to act, stops giggling and delivers a performance. 'You say you love me, well there's six billion people on this earth who can say that. But is there one who can stay with me?' Unfortunately it then cuts to Diana plonking away on a piano and then tearfully headbutting the music stand.

The decision not to depict Diana's Mercedes racing into the Pont de l'Alma road tunnel reflects the film's general cowardliness and lack of respect for the gravity of the story. We are shown Diana in the lift, mimicking with some effect, the now infamous and much-reviewed CCTV footage. I sense the impending race through Paris. The panic. The confusion. The disaster. The breaking news and the breaking hearts. No. Instead, we cut to Dr Khan sitting up in bed, gasping. Such is the strength of their bond. Either that or he gets sleep jumps.

Without seeing the ultimate tragic close of Diana's life – not the gore, but the chase and the symbolism of the car being swallowed into the tunnel - we are left struggling to remember the true feeling of horror and shock at what happened. But it's all rather too sanitised and cautious. The moment is impotent. As is the entire film. As my mum said on the morning of the 31 of August 1997. What a waste.