Before we all start throwing stereotypes around.
A solicitor, and therefore, the person they represent, is entitled to ask any questions they like of their husband, irrespective of the next person's relationship with the husband.
However, if, you are going through the courts, the courts may agree that certain questions are unnecessary or unreasonable. Go to court if you can't agree what they are/or what you think they should be.
Let's pretend, just for a moment, and OP, I'm not being personable to you here. But let's pretend...
The OP lives in a mansion, mortgage free, and has enjoyed, and will reasonably enjoy, a life of leisure for the rest of her days such is her financial set up.
The partner of the OP, pays 75% of his income 'reasonably' to his spouse. His income 'just happens to be' £100 per week. You don't mention numbers, so I'll be extremist about this.
So he keeps 25 pounds per week for himself. The partner is mighty fine about this because she's 'sorted' for life.
No judge, given the situation is going to interfere with this set up, even though, statistically, it's "unfair" statistically speaking.
Because, equally, the assumption can be made that the 'partner' can afford for the husband to pay 75% of his income. And that the mother of his children is relying on Tax supported benefits to support her 'genuine' income needs.
So, now you get the picture as to why, you describe yourself as 'his partner', that you're interfering with a pre-existing contract. Since you don't actually discuss the numbers directly, I hope I have illustrated to you the legal system that your 'partner' is married into.
As a partner, you have no rights. As a husband, he has responsibilities.
Yes, they can ask. You can also sit tight until a judge decrees. But if you're going to be 'it's not fair' about it all - have a little think: The wife is paying the home bills, your home bills are 'shared', or maybe not. All the legal system is asking is 'what can your partner reasonably afford to pay for his children'. So yes, they are also asking 'how can you reasonably afford to pay for your partner, her husband'.
That be the law. It sucks. But that be divorce.