ss wrote:

The last time I skimmed a joint in plasterboard I used a product which I
cant remember the name of but it was sold as an easier to use subsitute
for normal skimming plaster, although proper plasterers didnt think so,
it dried white and came I think in a 5 or 10 kilo bag (powder)It sanded
very easily and gave me a good finish.

Any guesses as to what it was.

Dunno, but Wickes do a ready-mixed jointing compound which sands well