OK, just been and tried this again (on my own this time), to summarise:
- The Travel Assist button on the steering wheel needs to be pressed every time you want it to be activated (it doesn't seem to remember that it was previously turned on.)
- Braking, or turning ACC off turns off Travel Assist.
- Simply 'resuming' the ACC doesn't turn Travel Assist back on.
- It does now keep the car pretty central in it's lane (as I am used to).
- With Travel Assist on the steering is noticeably lighter and dare I say a little twitchy (steering is set to 'sport' under the Individual mode), I'm sure I will get used to this but it's just an observation.
It also made no difference at all to the weight of the steering, there were no downsides to it being on IMO, which is why I never turned it off.
It made long journeys or commuting home after a really long day really easy and relaxing as the ACC / Travel Assist basically drove you home on autopilot.
Will we use it in the ID.3?
On long journeys yes i'm sure we will... but for it to 'work' (feel OK) you really need to let the car do the steering as much as possible, when you try to steer it it feels far too light IMO.