If the existing slates are laid on a concrete base and are not loos and there is no movement then you can pour self leveling compound .
If not then you need to remove and install plywood prior to your flooring .
Hope this helps

If you intend to lay over the existing tiles then you need to prep them with a self leveling compound - really easy to do, basically seal along any gaps to prevent loss of the compound, then simply mix the compound in a tub and pour over the tiles, spread with a trowel if needed. When this dries overnight the following day a sealing layer can be painted on ready for you to lay the lino

If there is any doubt as to the condition of the tiles and sub floor I would remove them and start again with ply or self levelling if on concrete

Unfortunately there is no simple answer, the correct way would be to take up the existing tiles, remove the excess adhesive and supply and install plywood sheets to the floor screwed every 10cm, once installed seal floor with diluted pva glue. You can now lay your tiles

Hi
A thin coat of latex self levelling compound should do the trick just be careful to seal all edges or holes under the bath etc or the compound will all run away on you. Alternatively take up the damaged tiles and ply line the floor before putting the new tiles down but this way is a lot more labour intensive and requires a good level of joinery skill.
Good luck

Hi...the cracks appear when is not fitted playwood..."do not put paint over rust "...Take out the old tiles . clean all well .fit playwood by 9mm first than a new tiles .A bit more investment but your bathroom will show very good .Thanks