Newbie alert!  Had to call the RAC out to my 2005 Focus at the weekend as i coukd just push the clutch pedal up and down easily without being about to change gear.  They've diagnosed a Master clutch cylinder leak. Based on him quoting rhe wrong price to fix it, i want to be sure he is right. Before I spend money on replacing it,(not myself) is there a way of telling if it is the Master or the Slave.   Thanks in advance.

If it was the slave I'd expect to see some leakage from the clutch housing - significant leakage if the pedal goes to the floor every time without disengaging the clutch. If there isn't then it's more likely a piston seal has failed in the master cylinder so when you press the pedal down the fluid just passes by it.