On the way home, I was sure that I'd driven through a speed trap (man dressed like building worker hiding behind a tree with what seemed to be a sophisticated camera). If it was, I'd already been caught, so I carried on regardless. I spoke about speed traps when I got home to others, but everybody said I was crazy.
The postman has just delivered a letter showing a photo of my car, saying I'd been caught doing 120kph in a 90kph zone - fine 400Baht. No problems - guilty as charged

I'm just writing this post to let you know they are out there (could have been caught by another hiding between Bangkok and Phetchaburi - waiting for that letter now

Also, for future reference, what is the speed limit between Hua Hin and Bangkok? My SatNav puts it very low - would 90kph sound about right? If it is so low, it will significantly increase journey times.