FYI anyone else interested, I spent far too long researching and got these conclusions:
TomTom
Best routing, and TrafficHD is brilliant (Live devices only). Bricking is still worryingly common, any update can brick the device. Software doesn't work with XP anymore. No longer supports custom itineraries in the latest nav devices. First big nav brand, and they've managed to retain their position as the 'default' sat nav in most people's eyes. Live services are the least robust, TomTom seem to be keener on removing features than adding them at the moment. They say they'll come back in the future...
Garmin
Live features only work if you have an Android phone, but there's no subscription charge. Lifetime maps are cheap, but supported by very subtle advertising. Routing is the worst of the big 3. If you're not fussy, the cheapest by far in the long run if you get a model with 'LMT' at the end (Lifetime Maps & Traffic). On a personal note, I find their graphics childlike. Live services are probably the most robust of the 3 - but be careful of data roaming charges.
Navigon
My preferred sat nav. Not well known in the UK, but well known on the continent. Very good routing, and will offer 3 routes before you set off. The only polite sat nav - prefixes instructions with 'please'. Have a tendency to turn off on occasion. Graphics are the most aesthetically pleasing of all of the navs. Live services are good, but some may involve an extra charge.
Ultimately I prefer Navigon because of the clearer map, and the choice of 3 routes before setting off. That said, if you don't want to set up custom itineraries and don't mind running the risk of a bricked device, the TomTom probably is the best choice at the moment.
) and Porsche will code it in and then record it as a proper fitted option on the car. This was done by my Cayman S owning friend and I was amazed at how accommodating Porsche was 
I reckon it could out run Tom and his mate too 
