Go to halfords and get some nylon and rubber lubricant and spray it on the channels,works wonders for smoothness.
Also as the above post says change the switch ,I swopped front to rear as I thought they were handed.
The new switch only cost £8 from the dealer and the window is now twice as fast!!
I have changed a regulator but I do not think that is the problem,visually check for broken teeth ,if they are worn change asap as they can kill the motor(v expensive)
Nice to see someone else from the uk!!!