If you can smell a bad odour coming from outside check that it is not your drains that are blocked. You may find that your toilet is blocked, the grid that takes water from your sinks has overflowed or there may be smells around the grid or inspection chamber.
Check if your neighbours are experiencing similar problems, if not it is likely that your private drain is blocked and therefore the responsibility lies with you. You may want to check with your insurance company to see if you are covered by your policy. If you are unable to clear the blockage yourself you can contact a specialist drainage contractor, plumber or reputable builder to clear the drain or carry out any repairs.