Brian,

I see a lot of patios made as you describe, with no obvious step, so it
has to be possible to do without having damp issues.

Thanks for that, and your other thoughts. The exact arrangement of steps
down to the garden level is something I'm still working on: in a wilder
moment it occurred to me that raising the level of the entire garden could
well be a simpler alternative, even with all the other complications it
would bring, though I suspect I'm really straying into the realms of fantasy
with that one.