Is it worth buying the external power supply for the Nanocom unit or could it be also powered by one of those powerbanks that are used to charge phones, laptops etc
Nah, I wouldn't leave it connected for weeks on end but the amount it takes is tiny.
Not only that, most of the features require the engine running anyway so unless the output from your alternator is less than a few milli amps then it will be fine.
They've had enough of your money selling the Nanocom!
Not yet, I've been working in Germany and Holland for the last two weeks, home tomorrow night. I might try to go for a look on Saturday but I'm off to the states for 2 weeks on Sunday so I think the family will want some time!