If you think The Mask of Orpheus has a dense libretto, try Purcell’s magnum opus of 1691. King Arthur is an opera without a plot and, in musical terms at least, Arthur without the King. The dramatis personae alone will mess with your head, for meet Philidel and Grimbald, Cupid and Cold Genius, He and She, plus assorted peasants, sirens and shepherds. There is no place in the sung material for any Royals of Camelot or Knights of the Round Table, although in the spoken text (not performed here) Arthur and Merlin do put in an appearance.

The confusion may not be librettist John Dryden’s fault. So censored, expurgated and rewritten was the then Poet Laureate’s 'wordbook' to suit the various sensitivities of Charles II, then James II, and finally the newly crowned King William III, that many aspects of Arthurian legend failed to make the final cut.

Paul McCreesh and the Gabrielis have been performing various editions of King Arthur for 20 years and it’s only now that they have alighted on a version in good enough shape to record. This week the personnel from their new two-disc set (Winged Lion/Signum Records) reassembled in St John’s Smith Square, minus only two of their number, for an exultant concert performance.

In a witty spoken introduction, the conductor characterised Purcell’s work as an example of British exceptionalism. “And we all know where that got us.” Later in the evening, to underline his point, the men of the Gabrieli Consort became a country bumpkin collective to sing “Your hay is mowed” as a full ‘oo-arrr’ caricature, and at the line “for the honour of Old England” they each brandished a Union Flag. To general cheering, the women and members of the Gabrieli Players countered with the EU insignia. In the evening’s only musical misjudgement, soprano Anna Dennis had to follow that uproar with the opera’s best-known aria, “Fairest Isle”. As a dignified celebration of Britain its placing certainly made dramatic sense but the abrupt shift of tone was discomfiting.

The singing by all seven soloists (they were joined by two non-solo colleagues to form the nine-strong Gabrieli Consort) was a marvel. Thoroughly confident in roles they have rehearsed in depth, even baritone Robert Davies who stood in for an ailing Marcus Farnsworth, they connected superbly with the audience through well-judged stage business and uniformly admirable musicianship. Highlights included a brace of gifts for tenor James Way: “Come if you dare”, with its rhythmic earworm at “the double, double, double beat of the thund’ring drum”, and the lyrical gem that is “How blest are shepherds”.

Soprano Rowan Pierce and bass-baritone Ashley Riches had Act 3 all to themselves as Cupid and Cold Genius in the opera’s wintry comic centrepiece; high tenor Jeremy Budd too, in resounding voice, was vocally stellar in his few spotlit moments. Dennis and Mhairi Lawson duetted sweetly in “Shepherd, shepherd, leave decoying” while the full complement of singers delivered a torrent of contradictory GPS directions in the brief but hilariously staged “Hither this way”.

The Gabrielis’ approach to Purcell’s zestful score was intensely musical but never reverential. They wore their virtuosity lightly yet their playing dazzled, with Jean-François Madeuf’s mastery of the intractable ventless trumpet a particular pleasure. Amid the prevailing gusto McCreesh was not afraid to let Purcell’s more sombre music ache with beauty, never more than in the exquisite Aire that opens Act 4. Elsewhere his King Arthur was raw, affectionate entertainment, with every singer off copy both in solo and consort numbers and with stomping, cheesy semi-staged business and orchestrations that smacked of the street as much as the concert hall. It all made for a joyous, earthy night of opera. Just don’t ask me what it was about.