Putting a return in the street field works for Address Book, but means it won't sync with my phone any more.
It's true that the name and number are not both needed, but in something like "Flat 99, 123 Any Street", both the flat number and house number are essential. I can always make it one long line.
The harder problem is how to get a locality or area field in between the street address and the city. This isn't just a whim of mine - there are many addresses where the official postal address, as dictated by the Royal Mail, includes an area. The only thing I can think of at the moment is to use the City field for the area and the County field for the city. Not ideal.
Thanks for the suggestion, though!