Hopefully this is the right place for a couple of observations on the PID Lítačka app mentioned above, the first of which might save somebody some hassle in the future.
I had bought a couple of 24-hour tickets on the app at reduced fare (I'm 61) before leaving home, having been quite happy with the performance on my previous visit. Arriving at the airport, I opted to activate one of these as I walked through the terminal, but it kept refusing to do so. Rather than suggest a lack of internet connection was the issue, it merely came up with a message (in English) that there was a fault with my ticket. Eventually I gave up and dug out a spare paper one from my last trip and validated that on the trolleybus.
The problem, I later realised, is that although I had already connected to the O2 data network, which fortunately does not charge for roaming in the EU, my phone was trying to connect instead to the airport wifi, even outside at the stop. This requires your email address rather than connecting instantly, and as I was only passing through I'd not thought to log onto it. So, presumably any British traveller intending to use the app but who does not have free EU date would have to take the time to log into the airport wifi to activate their tickets. If you do have free roaming, either log in to the airport or turn the wifi off!
The other snag was when I took a full-fare journey out of town to Únětice. I had bought the appropriate paper tickets from a vending machine, but for some reason I could not get it to go into the validator at the front of the bus. I seemed to be the only person trying to use it - virtually all other passengers were scanning their phones on the reader next to the validator. I stepped off to allow other passengers to board, but given some encouragement I tried again and this time got it to stamp. This somewhat unnerved me, so for the return journey I activated a single ticket I had put on the app as a back-up, which required the correct combination of zones for my journey to be entered. Now, I believe I did this correctly, but when the bus came it would not scan. I was getting no help from the young driver driver (there were no other passengers at this point) so eventually I gave up and paid cash. The fare was only 30 CZK (just over £1) so no worry about the lost money, but does anybody with experience of using the app for out-of-boundary journeys know what the problem might have been?