how much should my partner pay his ex

can someone advise me how much he should pay his ex
they separated almost 2 yrs ago and he has been paying £750 a month but has asked for this to be reviewed.
should he have to pay her maintenance in her own right?
she works 4 days a week and in clerical work.
he works full time and gets £23K/year.
their mortgage is £207/month and there is approx 4 yrs left on this.
they have 2 teenagers 20 and 18 yrs.
20 yr old is due to start his 3rd yr in uni and the 18 yr old will start uni in september.
they do have savings although he has no idea how much there is.
any help would be appreciated
thanks

Crikey if he is earning £23k per year £750 must be about 50% of his take home.

CSA rate for 18 year old until September would be 15% of his take home, adjusted if there are overnight stays with you, but other than that he isn''t obliged to pay her anything unless ordered to do so which is unlikely on his income.

Trouble is the longer he pays it the more he is proving that he can afford it!

thanks for that and yes it is half his income.
he has suggested going down to £400 and she is not happy and has said she has seen a solicitor.
i totally agree that the longer this goes on it looks as though he can afford and the further in debt we get.
he wants to remain on the mortgage but its £207. i understand he still wants to be seen to support the kids when they come home but it wont be anything like every weekend.
i understand he wants to support at uni and think he should even though they are getting grants and loans due to mum being a single parent (although my daughters loan wont be enough to live on at uni as our home is classed as having 2 incomes)

Personally I still think that after September £400 is a lot, especially if it is getting you into debt.

Is there a reason why he is continuing to pay the mortgage? Is his ex working and contributing?

guilt i suppose - not even sure the kids are aware he pays anything.
her comment about opening up a chat about reducing the money was "you dont contribute to their food or school trips" !

He must feel very quilty! Work out how much 15% of his net pay this plus half of mortgage if he can afford to.

Is this a voluntary payment or was it a court order - and was it spousal maintanenece or child maintenance ? If there''s no court order he doesn''t have to pay and if it''s child maintenance he will only have to pay 15% of his net weekly income until the youngest is out of full time education which if they start uni this year will be 31st August, 2013. I only know this as my youngest is out of full time education this year and his father has said if he needs help with uni cost he would have to apply for maintenance off him direct under the children''s act. Kids over 18 unless in full time education are classed as adults. So in theory if it''s CM he paying he won''t have to pay anything unless there''s a court order in place.
If he''s giving her £750 a month he''s paying far too much and if it was me and he''d offered £400 I would have snapped his hand off knowing legally he doesn''t have to pay anything!!
Why doesn''t he give his kids the money direct to them??