It appears that groundwater is undercutting the slab of concrete. I am guessing some sort of drain pipe will have to be installed and cemented over. 
 

Any other ideas on how to repair and does anyone know if a “competent” repair person in Pattaya

A you are looking for a competent person in Pattaya for these repairs, moving to the Pattaya forum for local knowledge.

How big is the slab and how thick? Also what part of town are you in?  I know of a crew that I've used a couple of times and always positive results, I can ask the foreman but he will want to know the specifics plus location, also do you have a Thai that can help you interpret as his English isn't top notch and you don't want anything lost in translation.

2 hours ago, bbko said:

How big is the slab and how thick? Also what part of town are you in? 

The slab is only about 25 - 30 mm.   Slab is only about 1 x 1 meterThere would need to be fill underneath the driveway slab but not replaced.  Cement would have to be shoved underneath where it is washed away.  Close to Floating Market and yes have a Thai GF that can speak to worker.