Just replaced light switches in downstairs hall and upstairs landing lights. Followed old switch but now the 2 way only works when the downstairs light is in the on position i.e. the power goes to upstairs to control switching. When downstairs light is in off position upstairs lightswitch doesn't work. Thanks

Are you USA?
I assume that when you say 2-way you mean a double pole switch. What we in the USA call a 3-way.

You say you wired it following the old switch. Check the location of the common screw. They are not always in the same location.

The downstairs switch is a double pole switch also isn't it?

Uk

Check downstairs switch. The two wires that are connected to the traveler screws at the downstairs switch should be connected to the traveler screws at the upstairs switch. The common screw (not a traveler screw) should be identified in some manner. Either a different color or marked with the word common. Again the old switch may have had the common screw in one location and the new in another location. Could also be a defective switch downstairs.