UK SERPS weirdness - Google testing synonyms, or something?

I have something weird going on in the UK SERPS for a site of mine that has been at a stable no. 1 or 2 for its main keyword/s for about a year now: for about 3 days now there has been a complete mixup of the page one results (my competitors and I have basically disappeared from the top 200), it's now got a ton of fairly irrelevant pages which feature a kind of synonym of the target keyword. However, this synonym is NOT really a synonym, it refers to a slightly different product that frequently gets confused with the product my site features. I don't think a lot of visitors looking for that product would be satisfied with the result they get right now.

I have had this happen maybe once before, but that was ages ago, when the site was still new, so I figured it was a Google dance.

But now I am thinking Google must do this for some reason, maybe it's a way to hone the algorithm by testing out similar terms to see if they are synonymous or similar, by looking at visitor click-throughs, bounce rates etc.

Weird though. Obviously the main thing for me is that my site gets back to where it was, because this is costing me money..!

Any thoughts, seen anything similar?