backpayment for bedroom allowance

hello,

i was recently told from CAB that i was able to claim for an extra bedroom allowance due to having support from my family sleeping over in the spare room through the week to support me with my health/MH issues and claiming PIP enhanced daily living (which i have done for the last 3 yrs, before i ever claimed for UC/ESA).

i do not remember being asked about this when i first made my UC claim in april last year and when i found out i was able to claim for the extra towards my rent a few months ago i contacted UC to ask for this.

they approved it and i've had 1 months payment so far with the extra allowance, which fully covers my rent now. the previous 10/11 months i've paid the extra money towards the rent myself which hasn't been very affordable. i asked them if i could have this backdated as no one told me i could claim this at the time and i wasn't really very familiar with what i could/couldn't claim for benefit wise as it was all new to me and i'd come from working full time and i'd had PIP this whole time.

they sent me a message in my journal today and said they'd just sent my backpay request off and it would go to a decision maker to decide if i'll be getting it and a message came back within 2 hours with a decision letter which said no i would not be getting any backpayment.

I have asked them to MR this via my journal but i am very confused. am i fighting a losing battle here? they have said i should have declared this within 1 month of my change of circumstances but i've been claiming PIP since before my first ever benefit/UC claim and i was not aware i was entitled to this extra bedroom allowance and never even knew what it was.

thanks for your help