Many years ago I saw, or read, an interview in which Alan Bennett described the difference in getting a play commissioned by the BBC and getting the green light for his first film project.

He detailed how the BBC would seemingly take forever before agreeing a script: putting it through committees, raising objections and queries before finally giving it the nod. On the other hand, the film company had no such qualms signing up his script without a moment’s hesitation.

Then came the rub.

Mr Bennett went onto say that having committed themselves to a piece, the BBC saw it through meticulously without another word. On the contrary, the film company very soon, and very often, started demanding changes: Do you need 20 extras for that scene; does that really need to be shot on the Forth Bridge – wouldn’t Hertfordshire do? He complained that the finished film, the mediocre A Private Function starring Maggie Smith, bore little resemblance to his original script.

Perhaps then it’s no coincidence that his latest film, The Lady in the Van also starring Maggie Smith, is a BBC Films production.

Now, BBC Films itself is not beyond reproach. I’ve been scathing in the past that they allow political proselytising to dominate their films all too often – the dreadfully sanctimonious Salmon Fishing in the Yemen springs all too readily to mind. But there was little danger of that happening with Alan Bennett. Despite being strongly topical, his writing talent is such he allows his storytelling to carry his piece with political and social themes as a strong undercurrent.

The Lady in the Van is a self-acknowledged mostly true story of an elderly itinerant who in 1970 parked up her van in the well-heeled street in which Alan Bennett and other luminaries lived. After a passage of time, she is invited onto Mr Bennett’s drive where she remains for many years.

The film’s themes revolve around society’s attitude to mental illness, the elderly, sexuality and family relationships. In the erudite hands of Alan Bennett it keeps the right side of being thought-provoking rather than preachy.

Maggie Smith is simply wonderful as the eponymous character. It would’ve been all too easy to overact such a role but Maggie Smith is far too good an actor to ham it up. She knows that a withering look or tilt of the head can speak volumes and she uses such devices to devastating effect. Alex Jennings as Alan Bennett has a double role playing both the man and his alter ego, the writer, often in conflict with each other. It’s a neat device which works well. There is much quality in the supporting cast who present interesting characters with whom Alan Bennett has great fun, often presenting them as stereotypes before showing latent depth.

Unfortunately the film has some pacing issues. Director Nicholas Hytner allows the story to lull on more than one occasion, the film meandering and drifting just a little. But he clearly has Alan Bennett’s confidence having directed the film version of The History Boys – most of whose cast appear in one or two line cameo roles.

Let’s hope Alan Bennett, now a sprightly 80 something shown riding his bike at the film’s end, and the BBC can make more material of this quality.