Mine recently developed the same issue. I tried lubricating the cables etc, but then realised the problem was elsewhere...
On top of the gearbox itself, one of the 'levers' the cables connect to (the upper one, nearer the front of the car) is connected to a rod that passes through in a 'sleeve' cast into the top of the gearbox. I applied a little oil to either end of this and as if by magic everything became lovely and springy again.
I'll see if I can take a photo tomorrow, but its a hard place to see. To apply the oil, I used a kids paintbrush on the end of a bamboo cane, and carefully managed to put just a drip in the right place. It may also be reachable through the wheel arch after taking the wheel off.