Strangley enough I did notice screw holes in the lid today but could not see the screws because of bits of broken render so will hoover them out and check. Many thanks.
Thanks for that Woody, puts my mind at rest knowing it is okay to do this, will make sure it is sealed, no smell coming from it though at present and no sign that it has ever overflowed. Thanks again.
Have discovered under laminate flooring in extension in property a drainage inspection hatch. Have not opened it but outside there is a new drain hatch in the floor about 10 inches across with an openwork grid cover and inside is another pipe which has a cover which looks as if it can be opened...