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The Killer Chorus (0.38-0.56)
It's the power of this sing-a-long section that ultimately defines Merry Xmas: another four-chord cycle but one with a supremely novel twist. This is the brain-teasing appearance of a rogue Bb chord that emerges as a left-field bIII in the sequence I-iii-bIII-V (G-Bm-Bb-D).
That hijacked harmony is not diatonic to the key of G, and would have no place in a simple major-key pop song that typically draws exclusively from the basic harmonized major scale. The Bb chord is, as musos say, "borrowed from minor" referring to how it belongs indigenously to the Parallel Minor key (G minor) but can be plundered back into a major context to add subtly menacing texture. In this way, the Bb gives the song some extra steel and expertly ensures it straddles the pop/rock boundary for both commercial success and artistic 'credibility'.

The Descending Bass Run Revisited (0.49-0.56)
Notice how, on the second of the two four-bar chorus cycles, the same Bb is accompanied by a re-hearing of the intro's descending bass run.
Not only does this create a neat structural symmetry, but it also illustrates how canny songwriters often use a song's intro to sneak preview an unexpected harmony that will later be introduced into the main structure. It is a way of subconsciously preparing the listener for a more challenging development ahead. [One classic example is the sprawling intro to The Beatles' Here There And Everywhere where McCartney also uses a Bb chord to prime the listener, in that case for an inspired bridge modulation after a verse in G.]

Holder's Harmonization (0.38-0.44 revisited)
Irrespective of whether Noddy wrote first the chords or the melody, these two elements clearly complement each other perfectly, with the chords providing logical harmonic support for the main melodic pitches of his tune.
The following table summarizes the novelty of the melody which notably shuns the popular pitch path of an underlying 8-7-6-5 diatonic descent (of the G major scale) in favour of a cool, chromatic twist that unfolds 8-7-b7-5.
Stripped to its bare essentials, that crucial b7 (the inspired F note on "every-body's") is the killer touch that makes the tune, while the similarly harmony-defining Bb supports it as the 5th of the chord.

The Parallel Minor Bridge (1.45-1.56)
The delicate mid-song interlude is more than just a dynamic breather from the upbeat action. Having finished the chorus cycle on that dominant D, Noddy hangs onto the same root but now delivers a startlingly novel D minor chord and a mini progression that fundamentally alters the tonal atmosphere.
Semantically, the moody minor-key setting transports us to the early hours of Christmas morning, reinforced by the reappearance of that F natural note (functioning, now conventionally, as part of the diatonic D minor scale) in the melody "do if he sees your". This is harmonized by another Bb chord which is now recast as a formal bVI relative to the bridge's temporary tonality of D minor.
This Bb also cues a powerful Aeolian rock ascent: bVI-bVII-I (the classic formula for the Bb-C-D run at 1.51-1.56) as we exit the bridge and awaken from our slumbers on Christmas morning ready for a verse back in the brighter, 'daytime' key of D major.

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