Just now, Andrew Dwyer said:

Located the vent on the tank, a blue pipe sticking up at the back of the house ( pretty much where Thais would make their outside kitchen ). Close to it is a white cap sunk into the concrete, about 4 inches o/d with a couple of holes in the top , presumably to unscrew it ?

Bingo 

I think your post about smell has just about cleared itself and think you should start a new thread on electrics -  rather than a mixed thread. 

Yes, you're right, water in the traps and a bit of grout in the bolt holes seems to have cured it, although it could have been lack of use causing it to smell, anyway seems good now but it it reoccurs then I've plenty of ideas.

I'll start a new thread in the sparky forum .

Thanks everyone for all the answers and suggestions, seems like I'm good to go right now but I'm certainly a lot wiser on the subject.
From the number of different suggestions it is surprising how many people have encountered the same issue.

Whoever said " farangs sh1t don't stink " was wrong !!

Re: the electrical questions, I'll open another thread in the electrical forum.

On 7/27/2017 at 2:46 PM, kannot said:

rarely see a vent pipe on toilets here for the gases, vented at tank sometimes

i live in a 30 unit apt , the gas bubbles up in the drains . i seal them with a stopper when not in use . showed the manager  the flame from the bottom of the kitchen sink (no j  pipes) when you lit with a cig lighter . she stared at the flame like it was a shiny new toy ...no response just walked away . i think she was trying to figure out how to use it for cooking .