I wasn't Para so didn't do 'P' Coy but I did do the French Commando course which is run on prety much lines (excuse the pun on 'lines')
We also did 'milling' which I didn't find to be a problem (we did it in the infantry too), however on the course we had to do the 'self righting' thing which you do while crawling along two ropes suspended 6" above foul stinking ice cold water. Basically, you let go with your feet and are expected to dangle for a few seconds and pull yourself up onto one rope and contiue pulling yourself along said rope to the end, easier said than done!!
It's a 'skill' which is perfected eventually, usually after you've been in the water about 6 times.