A gully that fills and empties slowly is usually just silted. A gully that stays full even after the visible debris is removed means the pipe beyond it is blocked, and clearing it needs access from the chamber downstream.
Chamber lids can be extremely heavy and should be lifted with proper keys, not a screwdriver. Dropping one on a hand or foot causes serious injury, and the metal edges on older lids corrode sharp.
If lifting the chamber shows it full to the top, the blockage is downstream of it and you need a jetter. That is the point to stop and call someone.