I have visited this country every few weeks for 6 years, worked here for 10 more and lived here for a further 10 years.
My work is in farming and I would travel to different areas many times atone location for a few days.    I am hands-on so looked in every nook and crannie at each location.

I spotted this creature just outside the front of my bungalow, on a cemented area, I rushed back inside for the phone ( camera ) but by the time I got back outside it had disappeared.

The insect was a creamy grey colour, about 3cm long 1 and half cm wide, not even half a cm high.
Its whole body moved in fine undulating waves like ripples on a pond.
I have never ever seen an insect like this in any country I have worked in / visited.

 

Sorry for the poor description but any ideas what this insect may be ?

Sounds a lot like the Asian Creamy Gray Undulating Rippler.  Harmless unless you're not native to the area.

Sure it wasn't a leech?  (Or "pleeng" in Thai)   ปลิง

They can be small, flat and grey.  Or enormous and black.