Pennies from Heaven (1978)

The one that changed everything. Six episodes of about 75 minutes each on BBC1 in 1978, with Bob Hoskins as Arthur Parker, a traveling sheet-music salesman in the mid-1930s, frustrated in his marriage in the London suburbs. It was the first Potter work where characters burst into full performances of popular songs, with Al Bowlly's voice all over the soundtrack, and the gap between those sweet lyrics and Arthur's grubby, desperate life is the whole point. It won the BAFTA for Most Original Programme and made Hoskins an established actor in Britain. Go in knowing it turns very dark.