Their way sounds correct to me.
The £100 is not taxable income its expenses, an expense is something incurred in your day to day job, for example your travel, client lunches, hotel accommodation etc (all depending what your job involves). These are not expenses that will be incurred when you are on maternity leave. If they were, you would be able to continue to bill for them and get them covered 100%.
Your income salary is the £400. This is what maternity pay is calculated on.
By claiming a proportion of your money as expenses and not salary, both you and the umbrella company are avoiding tax and NI on this money (please not I have said avoiding and not evading, big difference). Therefore on a day to day basis you see more of your money in your back pocket. However, this money is not seen as your salary.