Brian said

Just what 'bits of paper' do BCOs expect/demand from the
Structural Engineer's Calc's? Is it sufficient to determine
maximum load deflection appropriate selection (from finite range?);
or does the BCO expect a 'member of the beam-selecting club' certificate?

So far the plans I have submitted simply show a wall being removed and a
lintel being installed to replace it!
As expected BCO have approved the plans, but included a condition that more
detailed information be supplied.

I believe they will expect:

Calculations to support the beam size selected.
Relevant details of installation eg what will it be sat on (padstones etc),
how will it be secured.
Sufficient information about features in the vicinity - I have a lintelled
opening at 90 degrees to the proposed beam in close proximity.
Fire proofing measures to be used

My discussion with Tim has highlighted that my understanding is incomplete
as to what the joists are doing as far as the roof structure as a whole is
concerned.
I suspect that, if I submit calculations myself, the BCO will give them a
cursory glance, decide that I probably know what I'm talking about and OK
the modifications without looking at the bigger picture.
So if, through inexperience, my proposal is fundamentally flawed, the
problem may be discovered at BCO site inspection time (in which case
builder's costs rocket due to re-work) or may be discovered at roof collapse
time!

Phil