Bunty,
When my ex and I started going through divorce proceedings she was claiming that I should pay well in excess of what the CSA state I should.
My solicitor advised me not to contact them as once they had their claws in me I would never be able to wriggle free.
In the end, I took it on risk and spoke to them. I found that they stood apart from the rest of the divorce process in that they had a set of rules and they applied them fairly.
You are quite right that they will require 20% of his income to go to you as a baseline.
If he continues to pay the mortgage after the assessment, they will
offset this against his payments (don't know if it is in part or in full).
If he has staying contact of more than 52 nights per annum they will grant him a 1/7th reduction (2/7ths for 104 etc.), which I think is reasonable as they are under his roof and he is paying for them at that time.
If he incurs significant travel costs coming to see the children this may also be offset.
None of this is calculated until you contact the CSA. They in turn will contact him and make an assessment based on the evidence the two of you provide.
I set out the facts of contact, costs I incurred etc. with supporting evidence and was given a figure to pay which I felt was appropriate. I choose to pay through the CSA as I cannot trust my ex and wish to have nothing to do with her on a financial basis. You are in the lucky position where you can require he pays money to them, and not to you, so it is to the CSA that he must give his excuses if they payments are late. Even so, you can end up out of pocket as there will always be a delay if the CSA need to chase him up - unless they get him to sign up to a direct debit, which for some reason it is not possible to cancel??? (I needed to when I changed bank ac, although the CSA did take the money out of the right one).
As for the mortgage, it really depends on what is going to happen with the house. If it is going to be sold then logically the only money you should pay him back is the portion of his mortgage payments which are going to fund your asset (your bit of the house). If you are going to stay in it, I think you may have a problem as you will need to take on the mortgage (or get your own) and pay him out - giving you an even bigger mortgage... in a falling property market. My advice to both of you would be to change the mortgage (if possible) to an interest only one until all of this is settled. If you suggest that to him he may realise that you are only after what is logically yours and are not out to take him for every penny he has.
I hope you can sort it out as my ex and I started on fairly amicable terms and then she got greedy. I would suggest contacting the CSA, especially as they will take his mortgage payments into consideration (thus keep him a bit happier) and will reassess once the finances are sorted between the two of you.
Alfie