The Tetravan M.O.A.B. (mother of all brackets) is by far and away the best bracket we have found for worktop flaps, so much so that we import them from Tetravan in the US rather than settle for what's available in the UK. Solid, rattle free, level, and rated up to 95lb, they are the perfect kitchen flap or, doubled up, for a  fold up seat.
 
M.O.A.B. is 3" (75mm) wide and heavy duty, so only one bracket is needed for an up to 20" (500mm) wide extension. It incorporates a pressurized gas strut, which puts tension into the system and holds the table down during travel. Two magnets provide an additional latching mechanism, so there is no rattle whatsoever. A large spring-loaded release handle makes the bracket easy to use.
 
These brackets are rated to hold 95 lb/43kg. Two of them can be paired to make a fold down jump seat. We loaded one up with 270 lbs. and it didn't break, however we certainly don't recommend pushing one that far!
 
Included hardware: 4x carriage bolts, 4x washers, 4x nyloc nuts, 6x black 3/4" wood screws
 
Imported from the US.