My grandparents were visiting for the day,
I left it there for no longer than 30 mins.
So, what was your plan if you had a single(off-street?) parking place and two vehicles for the day? GSV shows plenty of unrestricted areas on Salisbury and Bruyn, was this the plan?
As any defence would probably be limited to mitigation, you need to present a compelling argument with all bases covered e.g. how did you know they were arriving early .. given your Nan's condition it's unlikely that she knocked on your door or that the driver left her...
You give us the full details and we'll try and put together a compelling argument as a minimum....
..not forgetting the other apparent occupier of your flat!