Can anyone advise me, do I need to prime skimmed plasterboard before I tile with a readymix adhesive and also i feelhave removed as much of the old adesive of the walls as possible with this be ok to tile over?

you don't need to prime if using ready mix addy, but it wouldn't harm if you did. With skimmed plasterboard, be aware that the safe weight limit for tiling is 20kg/m2, including adhesive and grout.

Prime or not? Depends on the type of adhesive you use as GG says. I use Ardex D20 and it is self priming. Cheeper alternatives might need priming.

Remove all the old adhesive, or as much a possible as bumps on the surface mean difficulties tiling, especially with larger tiles!

wouldnt hurt to prime the wall i always rpime newly plastered walls better to be safe than sorry

I depends if the plaster has been polished or not.
If not, then no need to prime.
If it has, you need to roughen the surface up,
then prime either with BAL Bond SBR (1:4 with water)
or BAL APD (1:1 with water)