The first was an engineered flooring joist it was made from 75mm
square timber frame with steel diagonal ties and occasional 75mm
square reenforcing. It came lengths up to 5M.

Wow, what was the name? I'd really like more information on this one.

I'm currently considering steel to get a 4m span under 100mm. A wood based
product would probably be easier to install than steel, provided it can be
approved for 30 mins fire resistance with only plasterboard below.

Christian.