Looking at the toll charges in detail I saw they were mostly to Aruba and Madagascar, and whilst no one was at home...

So I logged into my Fritz!Box 7390 and had a look in the Event Log, which showed someone logging in under the account "snapadmin" and making changes just prior to the fraudulent calls...

and then they set up their own telephony device "IP telephone 1"...

which was allowed connection from the internet...

I promptly ran Snap and they've been amazing in fixing the problem:
Apparently there had been some glitch with my initial setup, where Snap should have connected to my Fritz!Box, done their automatic configuration changes and reset the password from their default one to a randomly generated one, but for some reason this hadn't happened (been with them for over a year now) meaning the remote access account was still set to the default (and apparently known) password.
They've got it all sorted now, and are reversing the toll charges so very happy with their prompt support, although a little concerned that this vulnerability had existed (and could possibly still exist for other customers).