Anyone know if there is a convention/good practice when wiring
lighting circuits for halls/stairs/landings in terms of which circuit
they should be placed on.

Ie, should they all be on the 'upstairs lights' circuit, the
'downstairs lights' cct or split between the two depending on
location. Of course, the latter would mean that going to at least one
light switch would be power from both up and down lighting ccts. Is
that a problem?

Cheers
Rob