I had a chat with a builder who's doing the same thing to a house a
couple doors down. It's slightly different as they dont have the
dormer window, it's flat fronted so theres slightly more headroom on
the top floor.

Basically they were told by the council that the minimum size beam that
could be used in their case was 8x2. These were run parallel with the
3x2s and doubled up round the chimney (to support the beams directly in
front).

You were right in thinking the 6x2 purlin was supported by the studs so
they had to strap on another 4x2 beam to strengthen it. The actual
span of the purlin was in fact 3.6m (from party wall to party wall).

Unfortunately all this means I'll have a two level floor but without
taking out the ceiling below I'll have to accept this.

I forgot to ask the builder about bolting beams together, is there a
standard way of doing this (i.e. what size bolts, the spacing of these,
etc)?