At risk of sounding like a sourpuss, I can't help but think that this is something that most people who live in the UK work out simply by using web forms which autocomplete your address based on simply postcode and number.
There are two flat 1s in my postcode.
One place that would not work was the local council and so I could not get anything done, it took over a year of phoning up for this to be fixed. (of course the council tax database got it correct)
1 Example Close Somewhere Drive
But often when entering your postcode and selecting a house number, the address ends up as "1 Somewhere Drive" - which is a different place. The forms don't seem to realise that some locations should have multiple address lines. Fortunately in the UK, the postcode helps ensure our mail (mostly) arrives.