Thanks
I suggest a 400/230 volt transformer to power the PSU. You may need to
earth one leg of the 230 volt output to provide a neutral depending on
the requirements of the PSU.
Yes, but you would need to buy a 415v/230V transformer.
An autotransformer will do and may be cheaper than an isolating
transformer, but you are more likely to get 415V to 110V, so check if
your switch mode is dual voltage.
If the power is 50VA or less, you could use eBay item 170461492064
link to eBay: http://preview.tinyurl.com/ycnas4d
Connect the primary of the transformer between any two 415V phases with
a 2-phase MCB, e.g. http://www.tlc-direct.co.uk/Products/CM9806D.html
Cheers