I would say a disposable income of £2K. Don't forget you haven't included food, running a car (or 2 - or public transport), clothing for you & baby, nappies, not to mention a contingency fund for when something goes wrong with the house.
There is also the notion of them getting more expensive as they get older. The initial outlay for babies is a lot but their needs are very basic. You probably won't eat out much at this stage (too knackered and worried baby might scream) or go out much in the evenings (too knackered, breastfeeding, maybe).
And also, you need to look to the future a bit. Do you plan to go back to what you are doing in the future? In that case, would it be worth keeping your hand in? Is there an option for working part time? What you are earning is quite a healthy sum and, even with childcare costs, it would be worth working from a financial, long term career prospect.
However, money isn't everything. It is very stressful working and having small children and very hard to be everything to everyone and spread yourself so thin. You've got to look at your work/life balance and your sanity. When they are at school, you miss out on school stuff and potential support networks.
Personally, I couldn't have done it and mine are 14 and 9 now.