Thanks for all your support and replies.
My son was born on 12th June and didn't have collodian membrane or any ichthyotic condition.
There was/is still something weird with my or his sulphatase levels (I think) but I'm not going to worry about it for the moment because he's fine and I won't be thinking about having another kid for a few years yet.
It's possible that the collodian membrane (if it was even that) on my other (stillborn) baby was absolutely nothing to do with LI or anything else, we won't know possibly ever.
Thanks again for all your help - I hope these posts will help other mums-to-be.