We have just wired a new 2 way, 2 gang, switch plate, which works, but for one side the 'on' position is up, and for the other it's down, which looks odd and is impractical. Is this a problem with the new switch itself, or something we've done wrong? (These switches are the only ones that control these 2 lights)
Flip one side from L1 to L2
Your switches are intended for both single-pole and multi-way (multi-location) applications; as a result, both the "up" and the "down" positions on the switch connect COM to another terminal (either L1 or L2, depending on which position we're talking about), instead of having only one position that connects the terminals together, with the other position leaving them open-circuited.
So, fixing this is simple: turn the breaker off at the distribution board, switch one of the wires on L1 to the L2 terminal for that switch (not the other switch's L2!), button the box up, turn the breaker back on at the distribution board, and enjoy your light switches!