Amid the fuss that will accompany the 50th anniversaries of the first hits by the Beach Boys and the Beatles later this year, another significant half-century probably won't get a look-in. The Four Seasons reached No 1 in autumn 1962 with Sherry, the first of a series of hits that saw them briefly running neck-and-neck with their hipper rivals. There are so many smashes in the catalogue that Frankie Valli – 78 now, and the only original Season left – fills an entire two-hour show without recourse to filler.

"Rock'n'roll!" he shouts unexpectedly during Who Loves You – one of a clutch of songs from his 1970s disco renaissance – but there's nothing rock'n'roll about the proceedings. In the 60s, the Four Seasons, fronted by Valli's remarkable tenor-falsetto, epitomised the last stand of old-style pop-showbiz, and showbiz he remains to this day. Rather like the other famous New Jersey-born Frankie, to whom he dedicates a satiny I've Got You Under My Skin, he merges Italian-American masculinity and schmoozy charm. Along with the hits and his still-head-turning voice, that's enough.

A surprising proportion of the crowd are young enough to know Valli only as the subject of the hit jukebox musical Jersey Boys, which, he tells us, has revived his career. The actor who plays him is in the audience, and one feels for him: how can he continue to be "Frankie Valli" with any conviction now that he's seen the real thing? Despite being backed by four close-harmony vocalists, who do much of the heavier work and more or less take over on December, 1963 (Oh, What a Night), the real thing is still impressive enough to cause goosebumps. The era that begat Valli and his romantic songs is gone, but as he finishes with a joyous Let's Hang On, it doesn't seem very far away.