Hi
When I used the Unibond cement based adhesive (which behaves like mortar
and spreads on wall using plasterers trowel, but I dont know if its any
stickier than mortar), I put on about 8mm thick in 1 go with no problems
with the tile slipping.
They've set really well too- in fact stuck onto the wall like the
Rock of Gibralter! which is more than I would have got with the Unibond
3mm ready mixed.
I was planning to use a dryish mortar mix and hope it will behave the same
way as the cement based tile adhesive already used.
I think I'll have to try out a single tile first to check how it behaves and
sets, unless there are any more good reasons why this is going to end up
as a failure.

SledgehammerSteve