ENGLAND'S PINK FLOYD has been conveniently tagged a "space rock" group by many who write and talk rock'n roll. The tag's as good as any, in fact subtly appropriate for a band whose long, successful career has seemd to me a bit more nebulous and ponderous than profound. The Floyd drew a paid 19,000 to the Spectrum last night - they are likely to do so again tonight - and while their current show exhibits no upgrade of profundity, I think it puts an end to the ponder.

Floyd may indeed know what it's doing, so there's competence and some imagination to the quartet's blend of hard rock, ultrasonics and logically enough, an intergalactic theme. But most of it lacks an essential ingredient which often will spell the difference between good rock and what Floyd does: in short, Floyd doesn't swing, doesn't engage the body - not in itself a sign of mediocrity: much of Zepplin's "heavy-metal" music is the antithesis of swing, but there are compensations of technique and intensity, which can grip at gut level.

UNFORTUNETLY, Pink Floyd seems incapable of such compensation. Their material new and old, to judge from last night's performance, has evolved for the most part into a not terribly accomplished and often plodding brand of riff-rock. The vocals are passable at best; guitarist Dave Gilmour gets off a sickly minimum of stricking licks, while the rhythm section of bassist Roger Waters and Drummer Nick Mason is at least diligent if nondescript; and keyboardist Rick Wright - surrounded by his back of synthesizers and such - has a showy talent for concocting all manner of otherworldly sound, but his earthly musicallity is decidedly less compelling.

No matter, though; the Floyd has won its huge audience not so much by the power (?) of its rock'n roll as by the warp of its fiddling-about. In concert, the groups effective use of quadraphonic sound (i.e., speakers positioned around the concert hall) and incorporation of visual ploys are enough to hook most fans. Such was the case last night, but only after a turgid, torpid first half consisting of drawn-out new material that had an understandably numbing effect upon he crowd.