Sewer smell in loft and bathroom

get a qualified plumber to look at it,poor plumbing design or poor installation may be the cause,leading to traps siphoning out,back pressure from drains pushing smells through traps does soil stack vent through roof ?,do you have an aav in loft space ?,get a qualified plumber to check it for you.

Have a look at the soil & vent pipe termination in the loft. Does it have a bulbous fitting on the top? If not the automatic air vent has not been fitted. If the soil pipe continues up through the roof terminating above the roof it will not be down to a AAV not fitted. The internal Sv pipe starts from ground level working its way up with multiple different fittings, it could be a split gasket or a poorly fitted boss. If you was to cut a hole in the boxing where the soil pipe starts in the property you should be able see if there visible signs of water coming down the S&V Pipe. As any poorly pipe work or fittings will always leak. I hope this helps. Regards Darren.

Hello. Please do not take offence. It's always difficult to track down an "awful smell". If you have exhausted all avenues plumbing wise there may be another reason for this "awful smell"... rodents. I'm just saying it's a possibility that could be investigated by a pest control expert who may be able to recognize the "awful smell". Good luck, Bob.