I asked and was given the advice that balanced power transformers must be an all-or-nothing solution, that it basically offered its own earth and you should never mix boxes connected to the normal mains with boxes connected via the BPS. For ethernet I was recommended an ethernet isolator of the type used in hospitals as they didn’t know how good the normal galvanic isolation is.
Is this true?
It’s not exactly true. But it is highly dependent on the type of BPS.
Component sized ones that you might shove on your rack generally share the same earth as anything else. It all goes to the same wall socket. In that context there is no earth difference except the fact it may contain a shunt.
Circuit BPS that you would mount near the CU offer a more unified benefit to everything on the circuit.
The earth shunt is really key here but they are by no means common for every BPS. In the context of hospital equipment I’d say this is true. When you fit such a unit for an MRI room, you may want to isolate the MRI machine from other terminals in which case you specifically do want some things on the BPS and other not to retain that isolation. In other words, things connected directly to the MRI all on the same BPS. Things not directly connected but just in the same room, not on the BPS.
It is probably best to think of the BPS as a firewall. What things do you want behind it protected from the demilitarised zone outside?
If two units are connected directly to each other with no isolation, then putting one on the BPS and the other on the regular circuit may partially defeat the purpose of the BPS. But only partially and only if the BPS includes an earth shunt.
In the context of hifi and trying to remove DC offset, connecting just the offending unit is almost certainly fine. OTOH, unfiltered BPS are very benign and you might as well connect the whole hifi.
Just speculation, but I am willing to bet Naim use one, even if indirectly. Assuming they are on an industrial estate, it is almost certain that a high current BPS is fitted on the mains entering the site to isolate it from other factories on the estate.
I don’t know what kind of technical feedback there can be but I bring my experience here.
Respecting the correct sequence of power on, before the hicap and after the nap250, waiting about 5-10 minutes between one and the other, the hum that is usually audible from the hicap is almost equal to 0.
Perhaps a little long - but clearly safe. About 1 minute is enough, in my view.
I had transformer hum once on a Nait 5. Unplugged it overnight as I was going to return it to the dealer. Tried it one last time the next morning and the hum was gone (and never came back)
I don’t care whether transformer hum affects sound quality or not. It spoils the experience as a whole and I would NEVER put up with it, especially when you consider the price of Naim gear and how long this problem has been around.
Arrived today.
Having checked a few plugs the house voltage is 236 which mean precisely nothing to me except it’s well below the 254 figure mentioned by @GadgetMan. The plugs checked had correct led light sequence indicating no error there at least. Handy little gadget, thanks for the suggestion. 
236v is good & in the middle of the UK ‘nominal’ voltage range.
FYI - UK single phase electricity is currently required by law to be delivered at 230 Volts,
within a tolerance of +10% / –6%, that is, within the range 253 Volts to 216.2 Volts.
The value of 230 Volts is said to be the ‘nominal voltage’
FWIW - my house voltage used to be 247-250v & my transformers had a hum audible in a quiet room. Voltage has been lowered (I believe nationally) & my voltage is now 237-240v & the hum level has lowered to almost inadible.
I happy this worked for you. 
Glad you like it. Shame it didn’t find something glaring but it is great to use for many scenarios.
There are a whole host of fun gadgets but some are less affordable and wouldn’t get enough use to justify. One I’ve always wanted, but cannot justify, is an Earth potential meter. Anything less than 1 Ohm is considered okay under most national standards to provide a safety earth. But anything over 0,3 is pretty useless from a noise dissipation perspective. Of course that would have no bearing on this hum problem.
I’m not sure what else I can do now. Following advise from various forum members I’ve:
Added a dedicated mains which was in place before Naim items were purchased.
Used an unswithed plug on the above.
Bought a Matrix 2 power block and arranged items as per black bear’s recommendations.
Bought a double Naim Fraim to separate the brains and brawn.
Used only Naim cables.
6 Had all item installed by a Naim dealer.
The hum is still unacceptably loud on only the NACPS one of 3 (soon to be four) power supplies, I’m guessing it would have been deafening without the above steps. 
As mentioned in an earlier post my Naim dealer some time ago suggested stripping down the power supply to fit some pads under the toroidal transformer, this hasn’t been done yet due to the current situation.
At this point I’m open to any other suggestions.
You can get cheap earth continuity testers on the popular sites. The trick is knowing what to do if you find a failure.
Continuity is already provided on the device @0.0 purchased. Earth potential is a bit more clunky. Involves a device that build up a charge and then flushes it to earth. A bit like measuring blood pressure. How much over time gives you potential in Ohms.
Much easier to ask the sparky to do it. Earth potential is in their ready bag o tricks.
Well done for trying all those. The final thing you could try is a mains generator (not conditioner). As you may know, they are designed to create a perfect AC sine wave. I haven’t seen anything negative about these other than price, but try and get one on demo if possible. It may also be worth investigating if it can be placed at your consumer unit end to keep living room box count down.
Surely, one of the simplest thing to do is to try a portable Airlink BPS …
Worth a try, but a BPS can make the problem worse as it adds impedance, plus they can hum themselves, plus they dont fully clean the power supply.
As @0.0 has tried so many things, I was taking them up an extra level.
Simplest thing is to plug it in at a friends house.
it is true, dismantling the kit, transporting it to another place that might or might not have its own DC on main problems is very simple …
Or better, plug it in at the dealership.
Its a PITA but it will give you some kind of answer.
Hi @bruss, I should have added this to the list!
I did mention in an earlier post that I took it to another property to try, same noise.