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Never had this before, new toilet pan fitted about a month ago. Every so often after a flush the water drops below the back lip and air enters the trap. The trap is no longer whole and smells can come back.

No changes at all to the waste pipe, been in place 12 years from new.
Is there any way of fitting an anti syphon to the pipe behind the pan? Theres no pipework available, it goes straight from the pan into the concrete floor.

Try playing around with the water level in the cistern.

I put more water in, and it still syphoned, but an hour later it was used again and it didnt. I'll try upping the float level a bit more.

is there a durgo valve or is it a pre 70's house?
also, is it a cheap forieng toilet with a low 's' bend replacing an old type design?

No valve on this leg. Its a concrete house, the pipes are all horizontal across the floors and then vertical inside the walls. No access anywhere.
The pan has a deeper trap than the previous one, but I would not call it cheap.