You are a Mumsnetter... you will agree to a Gross salary in the contract
If the nanny wants to know what they will be getting roughly as take home pay, then direct them to www.listentotaxman.com which will do the calculations for them. Tax changes during the year, it can go up and down.
A professional nanny should not object to being paid gross, in my view. Vast majority of other jobs are paid gross, so why not nannies.
I am seeing a Nanny tomorrow and she has a little boy of her own age
Hmm... it's late isn't it Science hasn't come that far yet.
I take it that you meant that her son is close in age to your daughter.
Lowering the salary due to the nanny bringing their own child is something that gets discussed on here from time to time. There are usually two camps, those who feel there should be a drop in salary, and those who don't.
Personally, I feel there should be a drop as your DD won't be getting 100% attention.
The nanny with luck will have thought about practicalities - such as double buggy, highchair, sleeping arrangements. Also what will happen regarding food - much easier to cook the same food for both of them (that then means you are providing food for her child, thus another reason for the salary to drop).
As the nanny is live-out, the lowest the salary can go is National Minimum Wage. However, I would doubt you would be paying that low... but you may end up paying a couple of pounds an hour above it.
You need to find the right match in terms of the nanny... can you get along with them, does your DD respond to them appropriately, does the nanny have the same views with regard to raising children as you do. That sort of thing. Look for the nanny first - then look at anything that comes with the nanny. As frakkinpannikin says, try not to be rushed into making a decision.