Health warning for electrical engineers: the Pulse Electronics BV 020-5427.0 230 V to 18 V AC transformer gives out a very distorted waveform on its secondary side even under no load:
It should give a nice sine wave, with distortion less than 1%. Since the distortion is visible, it must be at least 10-15%. This makes it totally useless for the application I wanted to use it for, a mains waveform monitor.
I tried with and without load, and on different mains loops with and without other loads (including inductive loads), with no change. The electronics laboratory at my work actually has incredibly good AC waveforms due to careful distribution design, so I could have ruled that out before testing them, but for completeness I did. No change.
So, there we go: named and shamed. If you want a transformer with a nice waveform, look elsewhere.