We can use the same cable to bond water and gas by looping from one to the other, so long as the cable passes uncut through the first clamp.
BS7671 seems to consider a the main "through" cable to still be one single length so long as the join is made without cutting it.
By the way, just in case there is any confusion with my original scenario, the cable in question is not the main earthing leading lead as such but the main bonding lead. The main earthing lead here would be considered to be the one running from the panel's earth bar to the supplier's incoming neutral on a PME/TN-C-S system.
I did think about the shared cable situation a little more carefully though, and there is one thing which might need some consideration. If a bolted fault happened to occur at the sub-panel (unlikely, but possible), then until the fault was cleared by the 50A MCB at the main panel you'd have a series connection of two 10mm (phase and earth) across the supply. As the sub-panel is almost at the same end at the bond to the pipework, for the split second until the MCB tripped the potential at the bonding point could rise to almost half the supply voltage.
