Ah, I get your pointThe Building Regs stated: 'the requirements of this part do not apply to...a material alteration'. Therefore it would seem to follow that Part M does apply. (Which for the record I don't believe.)
I have to say that, I suppose because it is what made common sense, I was (essentially subconsciously) assuming that it meant that, since the requirements did not apply to 'material alterations', they certainly would not apply to 'non-material' alterations (i.e. those which did not worsen compliance status) - but, as you point out, that's not what it actually says.
In fact, what it 'actually says' seems to make very little sense at all (at least, to me), since it seems very odd that the requirement to comply with Part M1 goes away if one undertakes work which worsens any aspects of compliance status - probably the opposite of what I might have expected.
Maybe this is another example of one of those things which isn't meant to make sense
Kind Regards, John