Clutch pedal flat on the floor and pool of liquid under car?

One morning I found the clutch pedal flat on the floor. I could pull it back, but it had no spring to it and just pushed to the floor easily.

There was a damp spot under the vehicle, so I think some brake fluid leaked out. There are some drips under the bottom of the gear box.

I tried topping the master cylinder reservoir with brake fluid, as it looks to be a little low, but it's hard for me to tell, because I find it difficult to see a brake fluid line in the plastic reservoir. I opened the cap, but it has a yellow filter so I still can't really see in to check the level.

I cannot find any leaking along the pipes that go from the reservoir to the master cylinder and all along to the slave cylinder. The slave cylinder is quite hard to see, but I can inspect it from below and I can see no signs of leaks.

I also looked at the master cylinder at the top of the clutch pedal and can see no signs of leaking inside the vehicle.

A week before this occurred when starting off from first gear I felt a judder as I pulled the clutch pedal out - the vehicle shook a little as it went into motion. It did not occur on higher gears. On the last day the judder stopped happening. Then the next morning, seemingly overnight the pedal was on the floor with a stain of liquid under the vehicle engine.

What should I check for next?

The attached image shows the reservoir, master cylinder (at the top of the clutch pedal) and the slave cylinder (from below).