TheScullster wrote:

Considering cables which run as a loose "loom" between the floors of a 2
storey house, would these be considered as closely grouped?

Assuming T&E then unless the cables are clipped to form a single layer
then, yes, the /bunched/ grouping factors will apply to a 'loom.'

At points where they pass through joists they are touching, between joists,
some are touching and some have a cable or two diameters between.
If you believe that these do represent grouped cables, then surely almost
all installations are grouped (at least at the point where the cables bunch
to connect to the CU).

Near the consumer unit you will usually need to marshal the cables into
a single layer, or into separate bunches of two or three cables, to
prevent the grouping factors becoming too onerous.

Also I can reasonably argue that the downstairs power ciruit alongside the
cooker cable is lightly loaded

Definitely not. The standard socket circuits (per Appendix 8 of the
OSG) already include an allowance for diversity (based on the floor area
served) and you shouldn't derate further. (The diversity allowed
between circuits in order to determine the maximum demand is another
matter.) For a standard ring circuit take the design current to be 20 A
for each leg or unfused spur when checking the cable size required.

But what if a future occupant decides to run electric heaters in each
room?

That's why you can't...

I currently have a 6mm cable feeding a double oven and large hob. I don't
want to go to the hassle and expense of uprating this unnecessarily.
Taking diversity etc into account, it appears that a 51A rated load equates
to approx 22A for cable load sizing.

That sounds about right (first 10 A, plus 30% of remainder, plus 5A for
any socket on the CCU). That gives you the Ib for the circuit.

Any light that anyone can shed on this would be appreciated

Work steadily through the procedure in Appendix 4 of BS 7671 to
determine (or check) the cable sizes. Make sure you understand the
difference between Ib and In. For a domestic installation it's safe to
assume that circuits will not be subject to simultaneous overload, and
that can help a lot, as can avoiding the use of re-wireable fuses. Make
sure you're using Table 4D5A for T&E current ratings, not Table 4D2A or
ratings on wholesalers' Web sites, most of which are wrong.

--
Andy