At the moment it would be difficult to not pay the mortgage if you know your wife cannot afford it alone, you are liable for this debt too if it is a joint mortgage.
If you were going through the CSA and were still paying the mortgage you could apply for a variation to the CSA amount based on "Debts held prior to separation". You would need to provide details of the mortgage you are paying and repayments.
If you have based the child maintenance figure on how the CSA calulate maintenance this would reduce the figure you would need to pay.
Do you pay the whole mortgage? How much do you pay against the mortgage each month?
How have you calculated the £420, did you use the CSA calculator?
An applcation for a variation on the grounds of "debts prior to separation" will almost certainly reduce the CSA amount payable. By how much? this will depend on the level of the mortgage, the CSA use certain thesholds when making their calculations.
Cant remember from previous posts. Does your stbx work?
For your stbx to remain in the FMH she has to be able to afford the mortgage and bill etc with what income/benefits she earns/entitled too, compulsory CM and possible any sm she receives.
Spousal maintenance is payable if when you have paid for your reasonable needs (which would the same of your ex''s) and CM, you have any disposable income left.
If you move out of the property before your ex is in a position to cover the mortgage and indemnify you, you might find yourself liable for a mortgage of a property that her new bf lives in and because the mortgage is in your name, you will be the one who ends with a poor credit history if the house is repossessed
DONT move out unless your ex is paying for the mortgage out of her income and you know it wont be repossessed.