UC advance - repayments deferral.
Not getting a response, I nudged my Work Coach about it and they replied that I should call 0800 916 0647.
Doing a search for that number, I found a gov.uk page about debt management and the recovery of overpaid benefits etc. - it mentions nothing of deferral of UC advance repayments. Anyway, I called the number only to be told I need to call the helpline on 0800 328 564.
I managed to get straight through to my Case Manager this time (whoopee!) and she had repayments deferred for 6 months. A big thank you to @Hart86
Also, see this.