I have a problem. My 200tdi disco would not stop last week. the brakes were applied but my foot went to the floor. after pumping for a while they came back a bit but the pedal went all the way down slowly with some braking present. parked up and in morning brakes were as normal. drove for a while and same happened again even after pumping, no brakes. parked up, tried next day and brakes ok again!
I'm thinking could it be master cylinder seals as there are no leaks present.
How easy is it to change the master cylinder?

sounds like your full of air ,as it will raise to the top when settled so gives you brakes first thing in the day try bleeding first and if you have any leeks they should present themselves if no good then start stripping and replacing !!!!