I used some Slablayer to lay new paving slabs, following the instructions on the bag; although the mortar set firmly as a mortar layer underneath the slabs, it did not adhere to the slabs - there was a sandy layer on top of the mortar.
I emailed to Lafarge (also pointing out that the instructions could be better:
a) Should less water be used (instruction 3 is vague "sprinkle .. until the Slablayer is wet through")?
(b) How long should the Slablayer be let to harden (no instructions on the pack)?
(c) Would dilute PVA on the slabs help (no instructions on the pack)?
I had a helpful reply: "From what you state, it seems likely that the slab has sucked the moisture from the Slablayer, thus preventing the cement hydration and bond strength gain. To prevent this, either damp the slabs down a little prior to placement or wet the slabs thorughly a few hours after placement. An external PVA would solve this problem and guarantee a good bond, especially if there is little mechanical key on the underside of the slab."
I followed both suggestions, wetting the slabs and coating their undersides with exterior PVA (not interior wood glue PVA) before laying them on damp Slablayer - and it has worked.