@Fruitbat99 oh that's quite unusual, I think they normally do. It's both a supportive and preventative measure- I know our social worker said that she needed to check we had a mortgage (and could afford it!) and made sure we would commit to paying that over, say a gym membership (I laughed at her when she said that, it's why it stuck in my head). She then actually mentioned she had written to one landlord for a couple who had mentioned they weren't sure about their landlord letting them have a kid, and they might have to move and he had written back confirming they could stay. Her teaching from that was it a lesson in tenacity approaching authority figures and knowing the law about kids etc. She was a bit old school belt and braces though, and had a story for everything (I think some of them were made up because they so conveniently illustrated the point she was trying to make to us!). It's frustrating that this didn't happen for you, but it does leave a possible pathway for the 'get the social worker to write to the landlord explaining the situation' idea if they hadn't already committed. Not all landlords are completely heartless, and you never know the dynamics at play. Maybe they know someone who adopted and will hold off selling for a bit, maybe a letter from a SW might feel official and scare them because it seems like it comes from the council, maybe it might just given them something to tell their friends about how they helped a vulnerable kid and a single mum. Who doesn't like being a hero in the retelling? I mentioned this situation to my dad and he said gruffly that if one of his tenants told him that they were in your situation he would hold the house for them for 3 years "for the kid, you know?" But like I said, he's a bit of a soft touch. It doesn't really matter, it's worth a shot. Even having 6 months of certainty might be helpful to get your ducks in a row, or just the prior knowledge that you will get information in a timely fashion is better than the uncertainty you have now. Of course, additionally there are lots of complications because the house might not sell, there might be a chain, it isn't an easy fix, but the more knowledge you have the better.
Anyway, I suspect the social worker will want to protect the placement to reduce trauma, and if I were a social worker I would be thinking that whilst some movement in living might be traumatic, to me it seems like a smaller trauma than disrupting an otherwise appropriate placement. If it disrupted she would have to move into foster care again anyway, so in terms of difference, the main difference I can see is whether she has another different primary care giver or not.
I really hope everything goes the way you want it to in the end. Please do keep us updated!