I think this is a nearly impossible question to answer without more detail -
Do you have family?
Where is your office going to be?
What are your spending habits?
For somebody young, single, and highly mobile, it is probably a good salary. For somebody older and married, possibly with kids and an appreciation for the finer things London has to offer, I don't think 70k is going to go very far. As everyone else has mentioned, rent is a killer. So are the taxes, for that matter.