OK, I know about "aligning" the shift mechanism, but is this still the issue if the shift is smooth when the engine and gearbox are cold, and it gets notchier as it warms up?

Wouldn't have thought so, no. I've never had to touch the shift mechanism on mine in the 7 years/110k miles I've had mine. Gear change feels the same as it always has to me.

 

Maybe the gearbox oil is low and/or getting too thin once warmed up? Box capacity is about 2 litres from memory and the oil should be cheap from the dealer.