"Thanks everyone, next time she mentions a permanent arrangement I will say it won't work for me ."
I am one of those people who sometimes gets caught out on this, because I make the fatal mistake of needing to add on something else to "it won't work for me". An explanation, or an alternative suggestion, or even just some chit chat! Big mistake, as it gives the CF something to latch on to, to keep the conversation going and to turn it round!
So, knowing my tendency to do this, I now plan for it, by having my 'own request' at the ready! A request so outrageous that the CF will be so anxious to get away from my request that they cut the conversation as fast as they can.
For example, in response to 'sharing' the school run, I would say -
"No, that's not going to work for me. In fact, I'm really glad you approached me, because I was looking to have the opportunity to ask you if you could possibly take my DC to and from school for the the rest of this term?"
and, while their mouth is hanging open -
"Do you think?"
and, if they still haven't fled -
"Maybe a bit of next term too, but not much more than that?"