Yep, as mentioned Evans Cycles run "Fix It" classes:
There's a branch on East Dulwich Road as you head towards Peckham from Goose Green.
Brixton Cycles certainly used to do them, don't know if they still do.
Up in town, Look Mum No Hands on Old Street run regular bike maintenance classes.
And if you want to venture even further north of the river
there's this place:
] which gets good reports.