I can't comment on Nicholson's work because I haven't heard it, but I used to know the organ very well indeed half a century ago, back in the days when it was still quite new. It has had one or two things done to it over the years since its designer, Sidney Scholfield Campbell, died in 1974. At some stage the composition of the Great Mixture was changed and, although I believe that Jonathan Rees-Williams intended to return it to its original state, I do not know whether this was ever done. Also, I wonder whether the Solo trumpets were revoiced at some stage: there is a YouTube video in which they don't quite match my recollection.* In any case, they have been 'done' now. The original Orchestral Trumpet and Great Mixture can be heard in this performance by John Porter of Campbell's 'Impromptu' from his Canterbury Improvisations.
As the specification loudly proclaims, it is an eclectic organ. SSC believed in clarity of tone. Any suggestion of tonal mud was anathema to him. What mattered to him above all was the music. Musical lines and phrases were what he wanted to hear, not mushy effects (which is why he disliked Howells's music). The Windsor organ reflected this aesthetic. It's versatility was nigh on miraculous. You could play literally any repertoire effectively and could approximate at least vaguely almost any registration specified (though I never did discover a way of faking a Voce Umana for elevation toccatas). It particularly excelled in German Baroque music. The Great and Pedal diapason choruses to Mixtures were perfectly adequate for Bach fugues and if you wanted a bit more sparkle and incisiveness you could add in the Positive Principal and Cymbal and the small Pedal reeds at 16', 8', and 4'. Of course it wasn't perfect. The Swell mutations didn't quite cut it in Tierce en taille movements. They made a valiant effort, but their placing on the Swell was perhaps a compromise too far in that repertoire. On the other hand this allowed SSC to use them as solo colours in the psalms - and maybe that was always his intention. Swings and roundabouts.
The Great Cornet, which I was given to understand was cobbled together from sundry left-overs, did not gel well. It sounded too much like a Mixture rather than a unified pitch. I'm quite surprised that, apparently, it still survives in its original state. The Solo 8' Cor de Nuit was an indifferent stop (I was told that some of its pipes had once been an Open Diapason), so I can well understand why that has been replaced. I note that the Positive flues have all been revoiced. They were very '60s and full of chiff (has the revoicing eliminated that?), but it was a very characterful section with well-voiced, colourful stops. I am unimpressed by the discarding of the Great Blockflute in favour of an Octave Quint and the substitution of the 4' flute with an harmonic one. The original flute chorus didn't speak with the incisiveness that the Positive stops did, but couldn't that have been addressed somehow? The substitutions are a distinct departure from Campbell's concept.
The Pedal 32' flue was voiced quietly enough to balance the Swell Echo Gamba + Voix Celeste. It did so magically, but the volume could have been upped a little without detriment and I see that that has now been done – but why on earth replace the Swell stops with a Dulciana and Vox Angelica? If they are the usual, anaemic type SSC would surely have had something acerbic to say about that! The Swell Mixture was an odd stop. Its value was that it acted like a Plein Jeu when partnered with the French-style chorus reeds, but it didn't sound well without them. I can understand why it has been rehashed, but I wonder how much of the French flavour in the original Full Swell still survives. Because of the Great reeds, Full Organ sounded fairly English. If you wanted it to sound French you added the Solo trumpets, because the Orchestral Clarion had French shallots and proclaimed the fact very excitingly. The Orchestral Trumpet had English shallots, but it still had plenty of fire. These two reeds were balanced deliberately with a view to being usable in Full Organ without obliterating the fluework. If raising their pressure has spoiled this balance I would be most disappointed, but I am sure those involved will have been careful about this. Certainly there were times when one felt that the 8' trumpet could have done with a little more 'oomph', but it was never intended to be a Tuba and the compromise worked well enough.
One of the most impressive things about the organ was the independence of its Pedal department, which is why it is the largest division on the organ. The diapason and flute choruses balanced their counterparts on the Great and indeed the Pedal was completely independent of the Great at all dynamic levels. The Great to Pedal coupler was never, ever drawn. Conversely, the Gt and Ped. Combs coupler was never pushed in - and woe betide any visiting organist who left it so! SSC had set up the Great and Pedal pistons so that each one balanced its partner perfectly. You still had to use the Sw. to Ped. when Sw. to Gt was drawn, but in those pre-sequencer days it made dynamic changes so much simpler because you didn't have to be forever jabbing at the Gt to Ped. reversibles in addition to the Pedal combs. If you wanted to go suddenly to a quiet Swell, you simply hit Gt or Ped 1 and there were your Pedal stops (I think Ped. 1 was the 16' Quintadena + Dulcianas 16' and 8', but my memory is very hazy). I was roundly warned never to use the Gt to Ped. since doing so would make the bass line over-prominent. All SSC's organists knew this, including John Porter (who in his organ scholar days had helped SSC set up the pistons), but I'm not sure how long it remained understood after John died – certainly there are later recordings where you can hear clearly that the coupler is in use. Compare these to the tracks on Porter's excellent Priory recording.
Incumbents will always tweak their organs and what they do is certainly none of my business. I think it was this organ that, when new, prompted some criticism in The Organ from organists scandalised that it didn't have a 16' Open Wood – how can it possibly be proper 'cathedral'-type organ without one etc., etc. Ditto the Tuba. Of course they had completely missed the point. That was exactly the sort of organ Dr Campbell didn't want. He was aiming for - and H&H gave him – much greater versatility, an organ with fullness, clarity and ample pizzazz. But it does sound its very best at the console (because that's exactly what the good doctor ordered!) No doubt the day will come when someone insists on adding that Open Wood, but maybe space considerations will save it from that ignominy. As you can tell, I think that any attempt to mould SSC's instrument into something more 'symphonic' would be misguided, but, like I said, it's nothing to do with me. * According to an article in The Organ, April 1966, by Sumner and Campbell, the Solo reeds were originally on 9 in. pressure, but Roger Judd, in his book on the Windsor organs, states that in 2002 they were on 8 in. Campbell also says that the Swell was on 41/2 in., whereas by 2002 it seems that the fluework (only) had been reduced to 3 & 7/8. These two sources record other minor discrepancies in the pressures, but the 1966 ones look as if they were rounded so I don't know how much, if anything, was really altered.