It’s still somewhat of a mystery as to who gets to decide who is and isn’t a national treasure. For instance, after watching a recent, entertaining documentary on Billy Connolly, one of the contributors declared that the big yin is indeed a national treasure. It would be difficult to dispute, but at the same time, Connolly made a career out of using foul/colourful language as part of his act, which isn’t exactly befitting national treasure status in our books.

Then you have the likes of the Location, Location, location duo Kirstie Allsopp and Phil Spencer, both of whom we would seriously consider as NT material. But who gets to decide?

One person who has already got the national treasure nod, without any argument, is dame Judi Dench, who stars in this period drama, based on a true story.

Sitting at home one day is elderly Joan (Dench). She gets a knock at the door and when she goes to answer it, discovers it’s the police, and they’re there for her. They claim that she has breached the Official Secrets Act over 27 times, and that she it to be taken into custody.

There she is interrogated, for her actions in the past, when she was a young Cambridge graduate (Sophie Cookson), who became enamoured by a young German student Leo (Tom Hughes). But for them however, her relationship with the self-confessed communist supporter had repercussions that affected the entire country.

Although national treasure Dench is indeed the star of the film, it’s fair to say that Cookson, playing the younger Joan, does all the heavy lifting. It’s a role that no doubt Keira Knightley would have been chomping at the bit to play, only to be told that the part was now slightly out of her age range, as it’s a strong period piece with all the frilly trimmings. The role itself is less demanding, as the character is somewhat on the wet side.

Dench’s contribution is very little more than a slight turn of the head, eyes slipping to the top left hand corners to denote yet another flashback, where Cookson takes over to explain what exactly what happened. In all fairness, it's not much to do, but when old luvvie (which may well be one step below being a national treasure. Or not.) Trevor Nunn asks you to help him out, it must be terribly difficult to say no.

Now Nunn has a solid reputation for stage direction, but film is not his preferred medium, and it shows. This feature, although dabbling in some dark areas, just comes across as all too cosy. The early scenes present young Joan as a talented scholar, but naive in both love and life. There are opportunities for some risqué scenarios, but it all unfolds rather safely. And with the more modern scenes featuring Dench, they come across just as authoritative figures bullying an OAP. A national treasure at that.

Red Joan feels like a missed opportunity; the story on which its based is undeniably fascinating, and the bare bones of that are here, but Nunn’s film just isn’t as brave or bold enough as it should have been.

It doesn’t dent Dench’s national treasure cred, that’s probably not even possible, but this particular colour isn’t terribly flattering and doesn’t do her any favours at all.