Am I Entitled to LCWRA Back Payments?

Hi,

A bit of background first: I made my original claim for Universal Credit in September 2018 (last year) and handed in my doctor's note at my first face to face (17/09/2018). I was awarded Universal Credit with the first payment due on 20/10/2018. I was then sent form (U50, I think, the one where I explained my condition) which I filled in and returned. I finally had my assessment on 22/03/2019 and I was awarded LCWRA status, the first payment of which (after the mandatory 91 day period) is due 20/08/2019. This is fine, I've received notification in my journal that additional payment is due.

So, my question is this: Am I entitled to any kind of back payment (I have read that I should be entitled to LCWRA money from third month after I handed in my first sick note)?; when is that backdated to?; how long should I expect to wait for any payment to be made?; and, finally, do I need to do anything to claim?

Sorry this is a bit convoluted. Thanks in advance to anyone who can help