Their battle cry may be “Sontar-ha!” but this is more “Sontar-huh!” – so what!? The second episode, or rather Chapter Two of Flux, has potential. Sontarans en masse. Sontarans with a robust, tweaked design. Sontarans looming over Liverpool Docks and fighting in the Crimean War. Sontarans claiming Earth as an outpost (as Linx once did in The Time Warrior) and invading time (as they did in, er, The Invasion of Time). Sontarans played by quirky actors: Dan Starkey (in such roles since 2008) and Jonathan Watson, who I loved in BBC comedies Bob Servant Independent and Two Doors Down.

Despite all that, a lot of this hour-long conflict is pat and flat. Too much battle waffle and not enough action. The “War” promised by the title is a flash in the pan, although the brief CGI aerial shots are done well. I’m not expecting graphic massacre and screaming agony at teatime, but this war is anaemic. It has largely happened already or happens off-screen. The Sontarans, among Doctor Who’s most sadistic foes, have become comedic grunts, lousy shots and are easily thwarted. The potato-heads are still SpudULike not SpudULoathe.

Chapter Two runs 10 minutes longer than the first and the one that follows and, at its best, is intriguing without for a moment rising to levels of excitement. Intriguing, that is, if you’ve the patience to follow where this protracted yarn is slowly leading.

It gets off to a good start, though. The Doctor (Jodie Whittaker) comes to in a black-and-white wasteland, seeing a nightmarish crooked house in the sky. She’s reunited with Yaz (Mandip Gill) and Dan (John Bishop) on the corpse-strewn battlefield of Sebastopol, where they soon meet a miraculously un-grubby Mary Seacole (Sara Powell). A Sontaran commander clomps through mist on a horse. Yaz and Dan fizzle out of time in a blue haze, and the Doctor is unable to find any door into her police box.

Soon, she has one of her time-worn clashes with a military nitwit, who to her disgust (like her long-ago chum the Brigadier) eventually blows up the invaders. She also turns yet another historical figure into an ersatz companion. Good that it’s Mary Seacole when it could so easily have been Florence Nightingale. And handy that Mrs Seacole has so few wounded to tend, she can stand around nattering and act as an overnight observation scout for the “Doctoress”.

Fittingly, the heroes do the decent thing and exploit the Sontarans’ weaknesses in both time zones. We’re told there’s been a “temporal implosion”, but does that mean that in the 1850s the Empire of Sontar no longer overwrites Russia and China, and that in the present their forces are no longer all over the planet? Unclear.

Elsewhere, or to be precise in the Temple of Atropos on a planet called Time, Yaz and Vinder encounter prissy floating lights (akin to the Megara in The Stones of Blood). They’re led into the domain of the Mouri, “creatures who hold time in this universe together”. If you say so. Nice of them to do so. Funny we’ve never heard of them before. Vinder has yet to make much impression. No fault of Jacob Anderson who performs well, but I can’t be alone in suspecting this role was originally intended to be a starry return for Captain Jack (heavily set up in the last series and the New Year’s Day special) but hastily rewritten when John Barrowman proved unavailable.

As the Sontar-scenarios sag, it’s a relief when Chris Chibnall puts his newly minted aliens back on the chessboard. I’d almost forgotten them. Doggy Karvanista (Craige Els) – or Bungle-from-Bolton as I now think of him – is still sniffing after Dan, and their developing bond, despite initial antipathy, is amusing. I’m also rather taken with sinister siblings Swarm and Azure – excellent masks/make-up and delicious performances from Sam Spruell and Rochenda Sandall as swishy fiends just on the edge of camp.

On balance, I’m enjoying Doctor Who’s return to the serial format with tense cliffhangers. Each episode of Flux offers two. There’s a quasi-hanger before the title sequence, which would work better with that “Peeaaoowwhh!” musical sting, instead of a muted segue into the sludgy current version of the theme tune. The closing cliffhanger is also robbed of effect when, moments later, the Next Time trailer reveals the heroes alive and well in a completely different setting.

• Catch up on all the Jodie Whittaker adventures in the Radio Times Doctor Who Story Guide

Read more about Doctor Who:

• Doctor Who: War of the Sontarans’ 12 biggest questions
• Doctor Who’s War of the Sontarans featured nod to a classic Jon Pertwee serial
• Doctor Who: Flux reveals title of episode 4 featuring Weeping Angels
• Should Doctor Who’s Mandip Gill and John Bishop stay on after Jodie Whittaker leaves?

Doctor Who continues on BBC One on Sundays. For more, check out our dedicated Sci-Fi page or our full TV Guide.