How much should a single parent earn to live in london?

I am 40 and have a 5 year old daughter. Have worked in marketing and the arts for 15 years, currently on almost �50k in the north but would like to move to London to be near family. Got offered �40k at quite prestigious organization but didn't think that would be enough for me to live on. I appreciate that I wont be able to live like a king but I also dont want to live in a 1 bed ex-local authority on the outskirts. Thoughts?