I think I will go with the series r/c network.
You are right, I don't want to muck around relying on current gain. Basically with series the cap just needs to be big enough, so it is easier to size.
I will double check the pulse is long and large enough and add a good margin.
the latching relay would be better if i am sure it would always turn off, but it would mean I would have to put too much effort into working out, then explaining the consequences.
thanks david and dannyf, and the others.