Thanks
I drove through the Osoyoos/Oroville border crossing and it went very smoothly.
I was told I wouldn't need ESTA but would get an administrative (?) refusal. When I arrived at the US border control he advised me to pay $6 each for a 90 day ESTA. This would allow me to wither enter the US or turn around straight away and re-enter Canada. To not take this option would mean a wait of ~2.5 for paperwork, so I took the ESTA option. the immigration officer went through the card with me and told me what to write fro things like port of entry, address im US etc - very helpful. We went for lunch and headed straight back for the border.
The immigration officer on the Canadian border asked a few questions (nothing tricky or intrusive) and asked us to wait while our paperwork was processed. Within 10 minutes it was done.
Luckily for us it was very quiet both ways so that made a huge difference.