Quote: ...I have now spent a lot time trawling through the Regs and am hard put to find where it forbids the use of flexible cable for fixed circuits. To the contrary...
...Regs 17edition  (page 98) (521.9.1).  A flexible cable or flexible cord shall be used for fixed wiring only where the relevant provisions of the regulations are met.
Exactly.
And, because its use is NOT a deviation in itself,
no recommendation code should be used (provided, of course, that it satisfies the requirements of the regs). It is incorrect to comment on preferences, myths and fables in a PIR; all observations must be supportable by one or more actual regulations.
You said,
"as it was a flexible cable it was failed out right.". I would be interested to hear how your 'competent person' justified that assessment.
Quote: I haven't found anywhere in the Regulations, Yet, that prohibits a stranded and a solid conductor in the same terminal.
And you won't.
But you should consider the requirements of Section 526, especially 526.1 and 526.2, items (ii), (iii) & (iv)