Unless this lintel has to carry unusually high loads then a standard cavity lintel should be fine.

No floor joists should bear directly on these lintels though. (i.e. no point loads)

Also make sure there is no high load point in the wall above (steel beam bearing etc).  This may not be obvious visually.  We have built properties where massive steel beams have been concealed within the floor space so the walling below is carrying a VERY high load. Just beware!

What make have you seen that looks 'flimsy'  

None of the ones I use are flimsy:

I/G Expamet
Catnic
Cougar

[quote author=tensheds  link=1113339029/0#2 date=1113380632]

There are certainly no hidden bits of metal and there will be a coarse of blocks and and a row of bricks before the joists get picked up - presume that should be OK!

[/quote]

Yes, that's fine.  Exactly what the lintel manufacturers recommend.