whats the best thing to apply to bonding and hardwall undercoat plaster before tiling apart from a multifinish top coat skim as i have heard adhesive cant go straight onto this surface? is there a primer or pva of some sort?

bonding or hardwall is not a suitable surface to tile onto imo
skim the walls 1st then prime . i.e mapei primer g

what size tile are you using .as there could be a weight issue , depending on size of tile and what material they are
i.e porcelain

don't use PVA ( use the search facility to find out why ) use an acryllic primer, with large format tiles,use a powder based single part or rapid set adhersive

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Natural stone specialists mosaic, porcelain and ceramics