@Hannah378
From the host's point of view........hosts do not recieve any kind of payment until the intended check-in date so the host has recieved no money whatsoever from your cancelled booking. Any money paid by the guest for a confirmed booking is with ABB until the day after the (intended) check-in date so technically the host has nothing to refund since the host did not recieve any sort of payment regarding your specific booking.
The 50% that the host will get for the cancellation will be paid out by ABB to the host on the intended check-in date of your cancelled booking, which by your post I assume will be about 8 months from now. No host is going to refund money from their own pocket to guests asking for an additional refund thinking they will get the money back from ABB several months later.
While I understand "why" you are upset, the host is not to blame (at all~) for any of your problems or frustrations.
Also...... if you read the cancellation policy carefully it says.....
"Accommodation fees (the total nightly rate you’re charged) are refundable in certain circumstances as outlined below."
For a 50% refund of accommodation fees, cancellation must be made 7 full days prior to listing’s local check in time (or 3:00 PM if not specified) on the day of check in, otherwise no refund. For example, if check-in is on Friday, cancel by Friday of the previous week before check in time.
So regardless of how much the guest paid upfront, the 50% refund is based on the TOTAL sum of the reservation. You said "We paid something like £1200 and the total due (including the £1200) nearer the time was £2100." which means your accomodation fee is £2100 of which you paid £1200 as a sort of downpayment/deposit to secure a confirmed booking. According to the wording a 50% refund would be half of £2100 - it seems pretty clear to me.