Expat Status and Child Support Payments

I'm currently doing a trial psoting overseas in Sadi Arabia. I'm still on the UK payroll at the same salary, and subject to UK tax etc, so no change in CSA payments - currently by agreement with ex, who is the primary carer.

I'd like to know what happens if I accept a permanent posting, which includes tax free salary and benefits, etc. How is child support then calculated? If it uses the current formula, based on take home after tax, NI and pension (none of which is taken out in Saudi) then that amount would remove the financial benefit for me to be an expat. In addition, the ex (remaining in the UK) would also simply give up what little work she does do, as she'd live comfortably off of me. Nice, but really a very poor example to my children, who I'm trying to raise to see the benefit of hard work as they grow up.

Also, unlike the present arrangement (where my salary is taxed, etc., and ex gets money after tax) if she receives money made overseas and then brought into the UK, without any tax being paid, surely that becomes taxable income - who pays that.

I can find no help on this anywhere, yet believe it must be the situation for many people working as expats. Anybody come across how this works before?