Interesting...

So it repeats the signal its picked up from outside, and re-presents it inside the car so any regular device with GPS will simply think its locked onto a good satellite signal?

Exactly. L405SC just arrived... still working everything out!

For those that are interested...

You can power the GPS re-transmitter via the USB maintenance port next to the OBDII port. Possibly not recommended, but it's doing no harm.

What you cannot do is power the SatNav from this as it draws too much power.

As I far prefer TomTom to the built-in SatNav and I like things neat, I'll be wiring this in with a wire to the fusebox across the other side of the car. Working out how best to lay the cabling for this at the weekend...! L405SC just arrived... still working everything out!