Chris Martin and Johnny Buckland, Church of St John at Hackney, London Frank Turner, Academy, Bournemouth

A semi-acoustic session in a church by half of Coldplay made me realise there are some pearls among all that dirgy dross

St John at Hackney is a strange mix of the spartan and the sumptuous. Its cuboid interior features a glistening gold altar and ornate organ pipes, but bare walls and ceiling.

Huge arched windows, but only one of them stained. It's as though someone in 1792 set out to build the glitziest palace of Protestantism in all of London, but ran out of dough halfway through.

This sense of incompleteness is an appropriate setting for this year's Little Noise Sessions, a series of shows in aid of Mencap in which big stars play stripped-down acoustic versions of their oeuvre. Well, semi-acoustic: the first thing I see on entering is long-serving Coldplay guitar tech Matt McGinn, tuning up a decidedly electric-looking Telecaster.

"There's only two of them," says curator Jo Whiley before Chris Martin and Johnny Buckland take the stage, "so they're not going to play any Coldplay songs, if that's OK?" Of course, it's a tease, and when Martin sits down at his graffiti-splattered piano under a frankly awesome crown-of-thorns light fitting, and Buckland straps on that Telecaster, they're straight into their breakthrough hit "Yellow"... until the singer forgets the words.

Now, Chris Martin, as his sweary outburst at the Q Awards proves, does not take kindly to criticism. Well, tough. With their never-ending supply of donkey-plodding dirges, the bafflingly superstellar Coldplay, while not the "shittest thing on the planet", and while neither as culturally damaging as Oasis nor as irritating as U2, are nevertheless ruthless enforcers of the tyranny of mediocrity, the bland leading the bland. And self-deprecatingly referring to themselves as "the Shit Radiohead" tonight doesn't entirely get them off the hook.

That said, Chris Martin's Mr Nice Guy persona does carry him a long way. He's come, he tells us, straight from filming Graham Norton where "I felt like such a twat ... please don't watch it if you like Coldplay." When he apologises by saying, "Sorry, have I fucked up by swearing in church?", I can't decide whether it's really clever or really dumb. Buckland, in his wide-boy trilby, faces Martin over the lid of the old Joanna, and in their waistcoats and ties they look like a middle-class Chas & Dave as they rattle through a nine-song greatest-hits set before being whisked away by a waiting Daimler.

The freeform set-up allows Martin to vamp "Clocks" into a super-sped-up ending and get some audience participation going on "Paradise". I cannot deny that "Viva La Vida" is a truly great song, even if it took the Pet Shop Boys' cover version to make me see it. But current single "Charlie Brown" is a cavalcade of clichés ("they smashed my heart into" – can you guess? – "smither-eens"), and "Every Teardrop is aWaterfall", with its already-infamous howler "I'd rather be a comma than a full stop", reminds me why Chris Martin gets so much stick in the first place.

I go to roughly 150 gigs a year, and I rarely come across anything like a Frank Turner crowd. From the front to the back, the former Million Dead frontman turned rabble-rousing solo singer-songwriter inspires overhead handclaps and massed chants to every single song, and a phenomenal atmosphere of togetherness and unity.

Four albums into his solo career, Turner's popularity seems to be approaching a tipping point where headlining festivals wouldn't be too much of a stretch. It feels like a religious convention in here, and I'm the only Turner agnostic in the house: even the bar staff turn to one another and nod "I love him!"

White shirtsleeves rolled to the elbow, he and his band specialise in fist-pumping singalongs, many of which attempt the same combination of sentimental nationalism and socialism as William Blake and Billy Bragg. "Sons of Liberty" namechecks Wat Tyler, while another anti-establishment anthem features the gleeful chorus: "There is no god, so clap your hands together!" Arguably, as a posh boy who studied at Eton alongside Prince William, Turner shouldn't be allowed to get away with this. But Tony Benn was an aristo before he discovered socialism, and Frank's folkie credentials are certainly no more inauthentic than faux-Oirish Americans like Flogging Molly.

He encores with Queen's "Somebody to Love", but if anything, he's more of a surrogate Springsteen. "I sing simple songs about rights and wrongs", one new lyric runs, and that's about the size of it. Before another new one, he informs us that, "The end of this song has got a singalong section." Do any of them not?

"I try to do things that bring people together," he explains, "not push them apart." I still don't quite get it, but I wish I did.

Next Week:

Simon Price catches Sinead O'Connor on the comeback trail

Rock Choice

With his superb new double album Hurry Up, We're Dreaming to promote, Anthony Gonzales (above) takes his electro-dreampoppers M83 to London's Heaven (Thu). Meanwhile, Swedish quintet Little Dragon, whose singer Yukimi Nagano has been all over recent work by Gorillaz, play Brixton Academy, tonight (supporting Friendly Fires); Ruby Lounge, Manchester (Mon); The Arches, Glasgow (Wed); Thekla, Bristol (Fri); Concorde 2, Brighton (Sat) and Shepherds Bush Empire (Mon 4 Dec).