right, I am thinking of using the "Dulux plaster sealer" to be on the safe side. Im still not sure about when i should sand any imperfections? before or after the primer coat?
The emulsion you have is not appropriate to seal the plaster with as it's the wrong stuff, So, either use a contract emulsion first and if finishing with the Dulux, make sure the plaster's dry or, use the plaster sealer, only if you're sure the plaster's dry and then continue with the emulsion.
A wide bladed filling knife should be good enough to de-nib prior to any treatment.