The switches are now wired...thanks to everyone for their advice.
I joined the two upstairs lights so they switch together from either of the two upstairs switches and then ran two lengths of 4 core between the two upstairs switches and used two intermediate grid switches to provide switching between upstairs and downstairs, so can now switch on/of both upstairs lights from all three positions (both upstairs switches and downstairs) and can also switch downstairs on/of from same switches.
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