ok, a bit of a self-answer.
The part the screw locates into can be slide up and down when two side-fasteners are released. I must have accidentally depressed it when removing the cover. Now i have adjusted it back up, I can get the screww to loacte and tighten.
Now all I need to know is, how to stop the overflow problem I started with!!!!!
It's not constant, but with every flush, the tank re-fills, then overflows for about 30 secs, then stops.
Any help with this original problem welcomed - there is an apparent adjustment on the float, but I can't get it to move (cheap plastic bits, again!!!!!)
pensioner idiot
December 2008