Plumber Needed

Not to belabor the point but your statement was “or you could use Caustic Soda and then replace the piping.” No comments were made about the safety of the product. You are right there are safety concerns to adhere to and one should read and practice these as per the label attached to the container. You are also correct about caustic soda reacting to human tissue. Being organic this should be a given. So drain cleaners can be hazardous to ones health if not used as designed. If used properly and with the respect they deserve the blockage could be cleared.
As originally stated I do not believe the floor will have to be broken up and the piping replaced if the piping is PVC (polyvinyl chloride) if caustic soda is used. Of course if you feel this would be the result that is your prerogative. Of course the blockage could be caused by other things as helpfully noted by other posters here.

have read through this topic and some interesting stuff here.
Have a blockage that is in the pipe that connects to both the shower and the sink in my bathroom (the loo is fine thankfully).
Have tried the plunger routine (have tried one with the other hole blocked and have tried 2, one on each hole with no effect).
Had the tank emptying chap round and got him to put his suction hose on the end - this seems to have created a small space for the water to drain away sloooooooooowly.
Have shoved 2 bottles of Mr Muscle down the offending orifices but no improvement.
there is a large down pipe from the floor before reaching the horizontal pipe so have found it impossible to get a rod in far enough to reach the blockage.
Current thinking is to take the lid of the tank and shove a rod through that end prior to resorting to digging up the floor which I REALLY don't want to do!!
Apart from Caustic Soda (which makes me nervous - shame on me), any other suggestions would be much appreciated
It's a big bore pipe so am really surprised we have such a stubborn blockage which must be made up of hair/soap suds (unless Thais do really strange things in bathrooms?)