I haven't checked the 405's electrical requirements against the RS part but the obvious question, to which it might be hard to get a definitive answer from the spec sheet, is "How loud does it hum ?". The description says 'Very low induced noise (hum)' but I don't know if this means that it doesn't couple stray 50Hz magnetic field into the signal circuitry or that it doesn't buzz of its own accord.
On the basis of past good experience with them I would recommend talking to Canterbury Windings
http://www.canterburywindings.co.uk/standard_range.html. They specialise in toroidal transformers for audio equipment and they do understand the need for negligible noise in all respects. Perhaps the TM105A or TM301A parts from their Audio Grade list would be suitable ? Just to be clear, I have no connection with Canterbury except as a satisfied customer. But I have found them very helpful to deal with.
Cheers,
GJ