No, I've only been looking at one-way fares, mostly on the KL website but occasionally on Opodo or ITA as well. And from when I started looking (early Feb) through into late March, the KL website was clearly showing a OW fare of either £39 or £38 for the midday flight.

A good suggestion, Aviatrix, to look at RT, and I now see I could do that on my preferred flight for £79 instead of the current OW £401. That would dominate the ferry/rail option (considering the time difference), but still not as good as I had hoped, with my expectations anchored by the £39 I first saw.

And, ExpatSomchai, thanks for the Jet2 suggestion. I don't think that flight was showing up in my earlier searches. On the date in question it currently shows with a baiting price of £22, but in reality that appears to translate into [2 x 22 + 35 (3 bags) + 7 (seat selection) + 10 (check-in fee)]/2 = ~£48 per person (plus whatever they charge to use the bathroom), and it arrives a few hours later than I would prefer. Still an option, though, and if that's the competition that motivated KL to offer a £39 OW fare for a while, all the better.

But what I'm hearing here is that I shouldn't expect to see £39 OW on the KL flight reappearing any time soon.