Good question. Would have to go away and number crunch for a while to come up with an accurate figure. Also, what do you count as "essential" outgoings? Would something like an optional critical illness insurance policy count as "essential" for example? Things like petrol fall in both the essential and non-essential camp.
Anyway, I'm possibly thinking too hard about it so, my extremely rough top of head guess would be somewhere between £1k - £2k per month on "lifestyle" expenditure. (for a family of four).