Maintenance pending suit

A brief background. I am a husband, my ex left the family home 16 months ago to rent a property. I reside in the family home with two children (16+19), one in full time education (16) the other seeking work. I received a letter from the ex's solicitor full of demands and threats (you need to pay x if you dont we will go to court and you will have to pay all associated costs etc). Ex makes no contribution to the family, no bills, no maintenance for children, rarely visits the children. My ex is now claiming they need an additional income of 1000/ month and intends on going to court seeking maintenance pending suit if I dont agree. The breakdown of the family income is that she claims to have a net income of 2200/ month (no evidence of this has been provided), I have a net income of 5100/ month but after paying all the bills etc I only have a surplus of 126. My view is that I already subsidise her income to the tune of 1000/ month by paying her half of the mortgage and not asking for maintenance payments, so in effect her income is 3200 and mine 4100, if I pay an additional 1000/ month my income will be 3100/ month and hers 4200/ month, yet I need to provide for two children. Is this a sound logic? I should add the requirement for an additional 1000/ month is because her parents have decided to stop paying her 1000/ month.

Maintenance is based on needs and ability to pay. If she needs more financial support her first recourse would be to benefits. If she did get benefits they would be reduced £ for £ by any spousal maintenance. You would need to be in a position to pay more than she gets in benefits from any surplus income for a claim to succeed.
You both need to provide evidence of your income and necessary outgoings. If you can do that a claim may be averted.
NB if it is a joint mortgage you are jointly and severally liable. This means you have to pay if she doesn't. It is usual for whoever stays in the FMH to pay the costs of doing so if they can afford it.

Hadenoughnow

I think my main concern is that if I dont agree to pay 1000/ month I will have to pay her court costs while they seek a payment. My solicitor was very much of the opinion that it is unreasonable but her solicitor states I i dont pay it within 7 days they will start proceedings and I will be liable for all costs. I should note my solicitor thought the letter was very aggressive, there are lots of instances where I should pay x otherwise I will be liable for all court costs when they seek x, she was very much of the view that we should come to a negotiated settlement and only involve solicitors if we cant. I am in the process of completing the form E, neither have actually been exchanged yet.

Hmm. In financial proceedings you each pay your own costs. A costs order may be made if there's been serious litigation misconduct during court proceedings or a reasonable open offer has been rejected. That's pretty rare now. You'd have to do something that resulted in a wasted hearing.
It's just sabre rattling..
They could try to apply for a legal services order but there would be a similar examination of finances to that outlined for maintenance pending suit. Unless you have a big lump of additional capital in savings I doubt an LSO would be granted.

Hadenoughnow