Well, I tried again today, with only slightly better results. The up train was OK, but my planned return on the 1400 from Waterloo was cancelled, again stated as due to a train fault, so reliability does seem to be an issue (although my sample is very small, three cancellations out of four services isn't great). I suppose that until retractioning, they're still using 54(?) year old EE equipment that began life under the 4-REPs, so maybe it's not surprising that it's become unreliable.
One minor odd thing that I noticed is that some but not all units seem to have dark-tinted windows. When my up train ran in, I could see into the front unit's coaches quite easily, but not into the rear unit's windows. On board the rear unit, it was evident that a tinted film had been applied after the windows had been fitted, as there were small gaps at the edges where the film was cut slightly smaller than the glass. Does anyone know why this was done to some but not others?