Clutch Problem

I have a 2007 Skoda Octavia MK2 1.4L, 60,000 miles.

I have the car 18 months and I never had any trouble with it, am very happy with it.

A few days ago I got the timing belt changed at a Skoda Dealer and  they did their free 100 checks with no issues.

I was driving the car yesterday 10 days after timing belt was done and on the approach to a roundabout I was unable to change gear. When I press the clutch it feels normal till half way then after that it feel a lot heavier. When the engine is running I am unable to change gear.

What I'm wondering is:

What are the chances something happened when the dealer changed the timing belt?

I suspect either the slave or the master cylinder may of failed. Does anybody know the location of these?

Thanks in advance for any responses. I will keep you informed.