For most boyband members, leaving to go solo is not a sensible move. Four or five boys keep the magic of ‘something for everyone’ alive and many who stray soon return to the fold. Strength in numbers is key to success so merging two big boybands into a nine-strong ‘supergroup’ sounds like absolute pop heaven.

When ‘NKOTBSB’ aka New Kids on the Block and Backstreet Boys announced they would tour the world together, it was music to an army of diehards’ ears. Both US groups were huge – I mean genuinely massive – in their own heydays, and while fans and bands are about 20 years older, it makes nostalgia all the more wonderful.

And tonight’s show, the first of two London dates, is a real trip down memory lane, the crowd recalling the first time round with gusto. As images of each ‘boy’ flashes up on the screens in anticipation, the volume of screams suggests the old favourites haven’t lost their touch. New Kids’ baby-face Joey McIntyre, 39, seems to have aged flawlessly.

Opening and closing the set together, the groups take turns to hog the limelight, pounding out hit after hit. BSB bring Everybody (Backstreet’s Back), Larger than Life and The Call, and NKOTB serve up Step by Step, Hangin’ Tough and a slightly cringeworthy but infectious Cover Girl. They throw in moments of playful rivalry, and, in the battle of the bands, New Kids are perhaps the muscular rough to BSB’s chunkier smooth.

All the hallmarks of a boyband's concert are stitched in here from the dance moves to the slick styling, the audience participation to the overt love for the town they are in – there are no girls like London girls, of course. These men are such pros. Four women are plucked from the crowd to be serenaded by BSB, and New Kids get among the audience, being grabbed and kissed along the way.

As one group exits the main stage, the other appears on the b-stage, and it makes for a consistent buzz. Everything is choreographed to the letter, even down to NKOTB’s Donnie Wahlberg flinging his ripped-off vest into the horde as large streamers are released around him. Everything works.

The thought of Take That joining forces with, say, Westlife to bash out the old tunes sounds ridiculous, but our American cousins can get away with being cheesy and romantic. This joint effort was a brilliant move, one that must be raking in the cash from merch sales alone. But even if contrived, none of the excitable 30-somethings here seem bothered about that.

When McIntyre starts crooning Please Don’t Go Girl as though he was that pubescent, high-pitched pin-up again, it’s obviously all a bit of fun digging up the past. “It feels just like 1989 all over again!” he says. “We see no reason for it to stop.” After this surprisingly lively 'comeback', I doubt it will.