My reception of the DCF77 timesignal last week does appear to have been a transitory effect. I was passing by the weather station last night and noticed that it was no longer receiving the timesignal.
Now, I didn't think (though I could be wrong) that VLF signals were subject to enhancements of that type. I'm wondering if a local noise source, which normally obscures the signals, was switched off over the weekend and restarted yesterday. I'll keep an eye for the reappearance of the signals.
I think my old TS940 receives well enough at VLF, so I may try receiving DCF77 using that - and seeing what sense I can make of propagation.