fiftyodd remember that on the 5:2 plan the 'fast' days are a modified fast, modified by the fact you're not fasting completely but are allowed to eat 500 calories on your fast day. The meaning of a fast 'day' is a calendar day, not a 24 hour time period, so from waking up in the morning to going to sleep at night you only have 500 calories, so once you add in sleep time either side of the fast day effectively you are doing a modified fast for approximately 36 hours, aproximate because it really depends on when you last ate the day before your fast day and when you start eating again the day after.
When you read about 16 hour and 24 hour fasts on the 5:2 thread, these are when people choose to incorporate fasts proper into their fast days. A fast proper is no food at all for 16 or 24 hours, usually (but not always) starting from finishing dinner the day before a fast day until either lunch (so 16 hours) or dinner (so 24 hours) on the fast day. After this 16 or 24 hour period they then eat their 5/600 calories.
It is not compulsory to include a proper fast into your modified fast day, it's an option many choose because some studies suggest health benefits may come from periods of proper fasting (note I say suggested, the studies are not yet conclusive) and/or because they find it easier to cope with a fast day if they postpone eating until later in the day to avoid awakening their hunger.
I see you're already with us on the latest 5:2 thread, so you probably already have the answer to your question now, but I thought I'd post this reply anyway just in case someone else reads your question. All the best with 5:2ing!
In theory that I right, but it wouldn't be the easiest way to start 5:2. I would recommend you have say 400 cals on top of your muesili for today, eat normally tomorrow then do a proper fast day next time. The first few fasts are hard until you get used to it, no point in making it even harder on yourself.