Does anyone know?

Nah. They both mean pretty much the same thing, except a "flat" is generally in a multi-storey complex and a "unit" is town-house type affair. But a flat can also be a separate building on the same block as a larger house, like a granny flat or a converted garage or caravan. Maybe you should start asking to see "apartments" and really throw a spanner in the works.
 
Yeah the reason I asked was because I entered two contracts in a street I know... know these blocks are completely identical but one was listed as a flat and the other as a unit... it made me scratch my head!
 
Depends which school you went to, as they say ;)

They're essentially the same thing, although strangely I would be more open to viewing a 'flat' than a 'unit' :) 'Unit' makes me think of holiday accommodation.
 
Unit is a much better term, I like to think I live in a unit.

And it'd sound weird saying something like Flat 6/22 Accacia Avenue.
 
I live in a flat. It's a vertical building, and I'm on the third floor of three.

The housing commission flats (80 storeys high or so they seem) in Flemington I can see from my window could never be referred to as units!