Ah c'mon...
I do AQA chem, but our assessment for CHM3 and CHM6 partly consist of coursework from a total of about 6 experiments.
What I mean is, we do 6 experiments like the one you're doing whenever it is, under relaxed conditions, with time to prepare knowing what experiment we're doing (about a week's notice), and our teacher takes the best marks from each area (design, implementation, analysis and evaluation) from the 6 that we do to form 1 pseudo experiment, equivalent to your assessed practical.
So this year, for CHM6, I have 29/30 for that part of the module, whereas you guys would have to get it near enough spot on in EVERY area on the day to equal my mark... sounds amazingly harsh to me... !
Having said that, because all the people that do the assessed practical get crap marks, our's get moderated down a long way (mine will probably go down to about 22-24...)
So, being given a few of days to think about the experiment style, not even the task, is not unreasonable I don't think. And let's face it, most colleges are going to hint like hell to their students, and the one's that don't are stacking the odds heavily against them.
Good luck, I wouldn't envy you suckers!