Bad smell coming from behind toilet

Hi there, it sounds like there is an issue with the seal on the soil pipe. It may be that the toilet has not been fully inserted into the soil pipe or the soil pipe into the waste stack causing smells to re-enter the bathroom. I would recommend checking to see that the soil pipe is fully engaged both onto the toilet and into the stack. Let me know if this helps Kind Regards

My first thought is that, if the smell is one you would associate with the toilet, then the soil waste is not properly mated. is the smell strong after the toilet has been used? Your fitter may have used a flexible connection which may have moved, or there could be a poor joint in the soil waste pipe. Your question can be resolved by an expert after a survey of the area, his may require an intrusive survey, where some cladding is removed to ex;pose the problem. The fitter should really come and remedy this issue when you have contacted him. I hope this is of assistance?