Determine if gulley is rain, foul or combined.

call a decent building contractor with a drainage background in his profile ,post the job on this website and you will get a responce,dont know where you are but if in west london then i may respond

Hi, lift a manhole cover on your property, flush the toilet and see if you can see water passing through, then poor water down an outside rainwater gulley and check for water passing through, it's a process of elimination. Or if you don't want to do it yourself, phone a good builder or plumber. Good luck with it. Regard's Paul