Tim, Matthew,
thanks.
I have abandoned the idea in the meantime.
Firstly, I read that these powerbanks have an internal voltage of 3.7 V, and they cascade that electronically up to 12 V, but incurring significant losses on the way. So, even if they say to contain 50,000 mAh @ 3.7 V, that figure may well dwindle to 40,000 mAh or less @ 12V DC.
Then, for the example product linked above, the output at 12V DC is limited to 2.5 A, and that will not power an installed VHF during "Transmit".
Re the fire risk of Li-Ion power banks, I think that is something quite remote, and I would not be too concerned about that. We all carry Li-Ion battery powered devices on us and do not really think twice about the inherent risk of that. My waterproof navigation phone has a 10,000 mAh battery, quite near to a power bank, and I would not leave it behind for the theoretical fire risk.
C.
P.S.
I read some more about "mAh". Disappointingly, the seemingly impressive "50,000 mAh" of the quoted power bank, multiplied by the internal voltage of 3.7 V equal an energy content of only 185 Wh, and if that is then cascaded up to 12 V, and counting in the losses incurring on the way, the usable energy content @ 12V is not over 150 Wh. Compare that to the theoretical 240 Wh energy content of the 40 quid 12V 20 Ah Pb battery, and the choice is clear.