Firstly, the retailer in question did not say that it was 'normal' for Naim amps to hum, just that some people believed they have a greater tendency to hum than other amps. I have since read other comments to this effect on other forums. I have no loyalty or preference for either brand as I have never been an owner of either brand and this is my first experience of them. I agree that my own experience does not mean that there is a problem with the amp per se, but another's experience of a Naim amp that does not hum does not mean that there is not a problem.
After some experimenting here is what I have found. The source is a Squeezebox Touch (modified by Fidelity Audio) through a Keces DA-151 DAC. I am using a extension block and power leads by Clearer Audio, QED Silver Anniversary speaker cable and Tannoy Sensys speakers. With this setup the 5i hums and the K2 does not. I tried plugging the 5i directly into the wall socket rather than the extension block and it still hums. I tried different connector cables and it still hums.
Finally I tried changing the source to an old Sony CD player connected directly to the 5i and the hum disappeared (or at least was greatly reduced). Connect it again to the Squeezebox/DAC and the hum returns. This would lead me to believe that a) the 5i is not a faulty unit as it would most likely hum whatever the source if it were, b) the combination of Squeezebox/DAC and 5i causes the hum, c) the Squeezebox and DAC are not faulty as they do not hum with either my existing amp or the K2 and finally d) if a, b, and c are correct, there is some innate property of the 5i that causes the hum when combined with the SB and DAC.