Personal use on a tacho vehicle

Robaidh:
I’m potentially helping a family member move house. I can’t use my works lorry for it because access to their current house is to tight. However a pickup van and tri axel trailer fit just and no more. The van is fitted with a tacho.

Because it’s not for work/reward I don’t have to put my card in?

If I was to get pulled by the police how can I prove this particular journey is for private and not commercial?

Thanks

If vehicle under 7.5 GVW then no tacho needed for private use but I would put unit to - out of scope