I need to find the location of a manhole

If you have absolutely no idea where the drainage runs, most inspection chamber lids are metal (newer ones are plastic, but due to age are less likely to be covered) you may therefore fine by scanning with a metal detector. If you have an access point somewhere (no a gully) you can put a "mouse" on scanner on the end of a drainrod and scan from above...Jewson and other hire companies hire these items. It is possible that the Building control department of your local council may have drawings that show your private drains, if you are in a terrace try the environment agency as they may have plans for shared drains. Good luck

if there is not one visible then it may have been covered over ,to locate in these circumstances look for the outlets ie soil pipe ,gulley ect and follow a line to the possible central line of drainage good chance that at that point of intersection there will be a chamber