How about making a comment that you feel that the investigation into your complaint was effectively biased as it was carried out by your social worker's line manager?
It's almost certain that any decision made by the social worker actually came from the manager in any case.
It sounds like you originally made a stage 1 complaint. These are "informal" and carried out by the line manager. They have the advantage of not being counted by local authorities as they are not strictly speaking part of the statutory complaints system. If you go to the next stage - stage 2 obviously
- then it is investigated by someone from outside the management structure. It's not usually fully independent as it's usually carried out by people working for the complaints section and most carers I know who have used this system have felt that it is biased in favour of social services. Be that as it may, I have seen a number of successful complaints at this stage - it sounds like that is the stage you're going through now?
If so, you will have the chance to meet with the investigator who will ask you for the details of what happened, etc., again: usually before meeting with the social worker and manager to get their side of the story.