Which Luftwaffe units participated in the raid on Manchester on 11 March 1941?
There doesn't seem to be a specific answer to this question, although this is the general Order of Battle for the Luftwaffe around that period.
Responsible for the Northwest (Manchesters section) bombings were units of the Luftflotte 3 (Air fleet 3), supported by elements of Luftflotte 1 and 4, respectably.
When reading background information about the Old Trafford bombing (which, I assume you imply). I narrowed down the possibilities to the bombing corps of Luftflotte 3, which would be X.Fliegerkorps (X. Air Corps) Angers.
I'm afraid it's impossible to retrace which exact elements were responsible for the raid.