Shimy if they used a past paper it would be obvious as it would clearly be a GCSE paper and would say the date and time at the top of the paper eg May 14 2019, am.
I know there are very few past papers available for the new spec but I am really surprised if a school used anything other than a past paper for a mock. I see some schools do, but certainly for my subject (MFL) it's more or less impossible, certainly for reading and listening.
So beware of going along the line of “oh it’s a mock, it’s set much harder than the actual exam will be”. That’s pretty unlikely IMO.
If it is a past paper as I would expect, it will be an actual GCSE exam – most likely the school will use last year’s paper as this is one that students cannot access as yet so they won’t have seen it. So if it was difficult, especially any issues with timing, that’s what it will be like in the summer.
If students do find it hard, then that’s useful isn’t it? A mate of DS2’s really struggled with timing in last year’s mocks, he barely finished an exam because he just spent too much time on earlier questions and didn’t leave enough for the later questions, even though he knew the answers. He learned from that that he needed to stop work on the first question, even if he had a bit more to say, and move on to the next to avoid losing a whole bunch of marks – and did much better in the actual GCSEs.
WRT the poster who asked about timetables, yes they are the same for all boards, everyone will sit French listening and reading on 12 May and Eng lit paper 1 on 13 May. The content of the paper will vary from board to board. Timetables are available on the internet. Exams start in May, before half term. The first ones are usually French, computing, PE and RE but there are also some core subjects such as Eng Llit before half term. HTH