12c buyer

I cant figure out all that 12c buying a couple of days ago (Tuesday I think). Someone kept cleaning out the 12c line for +1m shares a pop for the whole day.

There has (sadly) been no sign of them subsequently and no substantial holder notice has been received. This strikes me as odd, as they must have taken a good few tens of millions of shares.

I guess it is possible it was a new entrant and they have stayed below 5% (80m shares).

How long does a company have to lodge a substantial holding notice?