When Joss Whedon brought together the different Marvel heroes for Avengers Assemble, the group outing proved to be a collective marvel, a sum surpassing its parts that was funny and characterful, inventive and exciting.

No doubt encouraged, Whedon and his team have gone in the only logical direction given the amount of testosterone in the package - bigger. More plot, more characters, more narrative ambition.

Diehard Avengers fans will have a field day with Age Of Ultron. For the rest of humanity, unfortunately, size doesn't always matter. And the sense now that we're watching a relentless behemoth in action is beginning to become wearing and a tad irritating.

The plot of this film once again sees the Avengers themselves create the threat that they must then save the world from; they're like a team of cowboy contractors, ensuring that there is always something to fix. In the last film, it was Thor's brother Loki, who turned his attention on Earth out of sibling bitterness; now it's Tony Stark, aka Iron Man, whose propensity to play God leads him to create an artificial intelligence to protect the world - but which promptly turns against him.

And so Stark (Robert Downey Jr), Thor (Chris Hemsworth), Captain America (Chris Evans), The Hulk (Mark Ruffalo), Black Widow (Scarlett Johansson) and Hawkeye (Jeremy Renner) suit up to take on mad robot Ultron (voiced by James Spader) and his two terrible twin assistants, Quicksilver and Scarlet Witch, East Europeans with a genuine axe to grind against Stark and with powers that respectively run wings around and mess with the minds of the Avengers.

All of the components of the filmmaking show a forward progression: the computer-animated action sequences and 3D effects are better balanced, resulting in some lovely sequences (my favourite is one of the first, as the team fight in slow-motion in a forest); the humour is in cruise control (with Spader contributing with a deliciously Machiavellian baddie, who thinks that his homicidal nature is a winning quality); and Whedon continues to work well on the human scale of things.

Particularly engaging here are the growing romantic attachment between Johansson's Natasha Romanoff and Ruffalo's ruffled scientist Bruce Banner; the Maximoff twins (nicely played, too, by Aaron Taylor-Johnson and Elizabeth Olson), who have suffered much and have the courage to reconsider their allegiance during the course of the action; and the revelation of Hawkeye's normal family life, something anathema to his colleagues.

If only the whole film could operate at this level; but it wouldn't be The Avengers without its sturm und drang. As ever, the extended sequences of smash and bash really do bring the eyelids down (how many times must we see cities torn apart by cartoon sluggers?), while the unheroic bickering has become exceedingly tiresome, not least that involving Stark - who has progressed from being integral to our enjoyment, to a bit of a bore.

Whedon tries to address the repetition, by adding Seoul and Johannesburg to the list of endangered cities; though this only adds to the overwhelming sense that these are superheroes the world could really do without.

And while the Avengers go to great pains to reduce the "collateral damage" of their misbegotten exploits, the result is a moral muddle, which reminds us of the sophistication of Christopher Nolan's Batman movies. Nolan dealt with questions of justice and vigilantism, the difficult choices involving who might live, who might die, and with loss and grief in a way that pushed the comic book genre towards profundity. It's a benchmark that's unlikely to be reached by the Marvel stable.