You could try some flexible filler but it’s unlikey it will last without stripping it off & re-enforcing. A crack that size is almost certainly along the line where the plasterboard panels join & either no joint tape was used or there has been significant enough movement to open the joint up. Do you use or store things in the loft; moving around up there can sometimes be enough to open up cracks in the ceiling.
Ahhh then; it could be a matter of no jointing tape as I said earlier but I suspect you have a problem with excessive flexing in the 1st floor joists. Problem is (if it’s original) the 1st floor of a cottage would never have been designed to take the weight of all the additional furniture etc. full blown living accommodation upstairs inevitably means; this can be a big problem on many house to flat conversions.
Assuming your upstairs neighbour isn’t a dancer, doesn’t hold regular parties or jump up & down on the floor, there is probably nothing you can do short of ripping up the floor & putting in additional or new & larger joists to take the extra weight.
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