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')