How can I fix my Renault Megane Brake problem?

I have a 2002 Megane Convertible, which has a brake problem.
The pedal is ok during normal use, the servo is assisting, but if I need to brake heavily, the pedal does not allow me to excert extra pressure on the brakes. The pedal seems to jam, and not continue travel as one might expect.
I have checked the servo by pumping the pedal until it goes hard, with the ignition off, and the pedal does then drop once the car is started, but at about 1/4 travel, the pedal seems to jam as of there is a brick underneath it. I have checked inside the cabin to see if anything was obstructing the pedal, but I could not see anything.
The car does have ABS, and I wondered if it might be something to do with that. The wheels do not lock up under heavy braking, but I think this is because whatever is stopping the pedal, actually stops the brakes applying hard enough to even begin to lock the wheels, and the need the ABS to activate
Can anyone help?