Perhaps someone knowledgeable about KLM pricing, or who regularly sees airline ads in the UK, can advise me?

My wife & I need to travel one-way between the north of England and AMS in mid-August. I had identified a good flight at a good price (~£40), on KLM LBA>AMS midday. While early morning and evening flights were considerably more expensive, the midday and other offpeak flights from LBA were all at that price.

I monitored the fare through Feb and Mar, expecting to buy in early April when the rest of our itinerary firmed up. But a few days before I was ready to buy, all the offpeak fares disappeared, and now all I see is ~£400 economy fares. This obviously isn't just because the fare class capacity on the flight in question was reached, since the minimum fare on any flight every day around that time is showing as £400.

So here's my question. Are there any informed opinions as to whether I will ever see the £40 offering again (I check every day)? Does KLM have periodic "sales," and if so, with what sort of frequency? Have they perhaps announced a new policy re offpeak fares? Should I just forget about it and make a Harwich-Hoek ferry/rail booking instead (since $1,300 for two of us to fly 60 mins. in steerage is just too exorbitant for me)?

Any insights or prognostications would be welcome.