Great pics, Steve
OP, sorry I didn't have to remove the handle back-plate so can't help with that. You did check that the cross-headed bar was correctly oriented in the actuator? If not, the two thicker tabs might prevent it from engaging on the handle back-plate side (even though it seems snug on the actuator).
As an aside, I'd recommend that owners get on to Brown and Gammons and pick up a few spares of CXD100080 - these are the little white plastic clips that secure the rod from the exterior door handle to the latch mechanism on the actuator. Think they cost about a fiver each (better looking at them than looking for them).
Cheers,
Karl.