How to put stock sony fw back on phone

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http://forum.xda-developers.com – I think/hope this might be an easy question. I have an xperia S that has bootloader unlocked and currently running a custom rom. In the next few weeks I may be swapping the phone with a family member. How would I go about getting the phone back on stock Sony firmware. I am not bothered about relocking the bootloader but I want to be able to put stock sony fw back on the phone so that they would be able to update the phone officially via sony pc companion. it's a bit :o as I can flash roms /kernels and unlock bootloaders on these things but have now idea how to get it back to stock (HowTos)