SSP and flexible working

We went over to flexible working for everyone about 18 months ago . It's an annual hours contract where staff have agreed minimum hours in peak months and have to post their working days a minimum rolling calendar month in advance. In practice, it is less complicated than it sounds because most employees have fairly predictable work patterns based round things like school holidays , exams, last Friday in the month off or say skiing in February and we don't always check whether they've updated their availability calendars .

I have recently paid out SSP to an employee with a predictable working pattern but who hasn't updated her calendar. However, we know she saves every scrap of time to supplement her study leave and would have expected her to be in on the dates if she hadn't been ill.

This is actually the first time I've ever done an SSP calculation and I'm a bit nervous so please bear with me . I know it's not a huge amount of money but I'd like to get this right.

If we expect her to be in ,are we ok to regard this as a normal working day?

Does it matter that she didn't have to be in contractually on that day?

Do I need to have evidence that she was supposed to be in and am I stuffed because she didn't complete her calendar?

Also, when I looked at the manual it said that if you came into work and did even a minute's work we shouldn't pay her SSP. Does this mean that I should pay her employer sick pay instead of SSP because she's logged in remotely and answered some emails from home? 

ps I think this is giving me a headache ...