Secondly I dont like your propsed beam extensions at all. The wood
thats there is sized for the half run,


Well no, because the original post said "All of the joists in question
are 65 x 175mm and the span from left to right is 2370mm." The joists
are presumably that size because that's what was needed over the main
rooms and they've used the same size throughout.

Indeed, a software application we know well ;-) shows a max deflection
of under 2mm at that length (assuming a 0.8kn/m uniform load with load
sharing, and C16 timber) - so well in spec. I an not sure if the 2.3 m
specified is the total length including the stairs, or just the existing
floor. If it is just the existing floor, and adding in the stairs added
another meter or so to the span, then it looks like the deflection is
still in spec.

2.3m is the total length including the stairwell so should be OK - cheers
for doing the calcs! :-)

Andy