If you've failed c1 I dont recommend doing A2 maths, I go to a college where people who got U's in Core 1 were allowed to continue to A2.. they are wasting their time, as they are struggling with A2 content, what you need to realise is there's another jump from AS to A2 like there was from gcse to AS.
I resat the C1 paper on Wednesday due to it being a really easy paper and wanting to gain extra ums.
I felt the C1 paper was one of the hardest I'd seen, but nonetheless it will still be the easiest exam I sit this year. If the paper was just a hard paper, the ums will be proportional, so if youve done the work and you have the knowledge, you have nothing to worry about. You only have to worry if you didnt.