The previous householders have re-used a 6mm^2(??) cable, (that I
presume was once used to feed an electric shower), to feed a power
shower pump and the (gas) heating system. (ie. low current draw
compared with the electric shower which I presume it used to feed).
The cable is protected by a 30A fuse in the consumer unit.

In order to make the wiring reach the power shower pump and the
heating system in the airing cupboard, they have extended the 6mm^2
with around 3metres of 2.5mm^2 using a junction box in the loft.

The current situation is clearly unsatisfactory as the 2.5mm^2 is
effectively (incorrectly) protected with a 30A protector.
I understand that the regs say that each reduction in cable size
should be appropriately protected, but I want to know if it is
acceptable just to place smaller fuse at the start of the whole
circuit in the CU. (ie. 16A).
[Working here on the principle that it just becomes a radial circuit
serving less than 50m^2 of floor area].
The alternative, if not, will be for me to replace the 3m of 2.5mm
with 6mm. Not a major issue, but I'm not sure about getting it into
the FCU's, (which incidentally have also been omitted by the previous
'installers')