TheScullster wrote:

I have had some plan designers in to draw up the general alterations, but
the details for the support of the joists was a bit sketchy. They mentioned
joist hangers to suspend the timber joist joint from a steel beam installed
in the roof space above the existing wall location.

I've seen this done using an I-section RSJ, where the ends of the joists
just fit tightly between the top and bottom flanges of the "I", with a
notch cut out on the bottom face to accomodate the bottom flange, so
that plasterboard could be attached flush beneath. This was in a fairly
old property, seemed perfectly firm and rigid but don't know if it would
be compliant with current regs.

David