What’s the gland plate made from? Paxolin or aluminium perhaps?

There’s nothing here to suggest an issue. Obviously we can’t see the installation. What in particular gives concern?

installed via a brass stuffing gland

Is that one (big) gland for all the cables, or many, one per cable?

If the cables are going through different holes in a steel plate, then that strictly doesn't comply with 521.5.1. With thin plates and modest currents the effect is pretty minimal though  - and the DPC for AMD4 suggests there might soon be another exception for switchgear and controlgear assemblies conforming to BS EN IEC 61439 series and circuits carrying ≤200A.

  - Andy.

or the multiple holes can be converted to a single hole by slotting between them an infilling with a non-ferrous material (dog boning).

  - Andy.

Individual glands ,through a galv plate, 5mm thick.

Then as above it does not conform to 521.5.1 (2nd para) and options are a) Replace gland plate with something non-ferrous (e.g. aluminium or paxolin) or b) cut slots between holes to break the magnetic circuit

Neither of which would be easy!

Double insulated and the brass gland mean that the cores will be some distance from the steel plate, which helps. It might be useful to know the typical load so that somebody cleverer than I can calculate the likely (heating) effect.

Agreed not easy to do retrospectively, but the OP asked a straight question of whether it complies, which it doesn’t.

I am anticipating a fairly hefty load (and even greater fault) current as the OP suggests 3 conductors per phase so I’m thinking transformer tails or similar, and 5mm thick steel is not insubstantial either…

If the installation is already complete and in use, then I would suggest measuring the temperature of the steel gland plate under full load. You may find that it is a non issue.

Still a non compliance, but if the temperature rise is modest, then arguably an acceptable non compliance. My own observations suggest hat eddy current heating is a non issue at 100 amps or less, and usually anon issue at up to a few hundred amps. There are, I suspect too many variables to calculate the degree of heating, hence my suggestion to measure rather than calculate.

Jam said:

Agreed not easy to do retrospectively, but the OP asked a straight question of whether it complies, which it doesn’t.

Indeed, but that of itself does not mean that any action needs to be taken.

broadgage said:

Still a non compliance, but if the temperature rise is modest, then arguably an acceptable non compliance.

I get what is meant by the above and agree. However, if the work is new then by virtue of the BS7671 definition of “non-compliance”, it would need to be attended to before certification was issued or, alternatively, identified as a “departure”.