Hi Chris,
The reason why I asked if it had campervan conversion is I had intermittent problems with my remote keyfob not working.
The buttons wouldn't work, no keyless entry nothing!
Then I would try another time and they worked perfect!?
After a lot of frustrating scratching my head and changing the key fob batteries, I found it only worked perfectly at night or indoors.
After a bit of research I found out that the Japanese cars use a different frequency on their remote keyfobs than the UK.
I have a solar panel on my conversion and the solar controller (MPPT) gives out a radio frequency that interferes with the keyless receiver on the Alphard.
I think there's other electronic devices about that give out a radio frequency, possibly in that London carpark, that would interfere with your Alphard but not and British sold cars maybe?
Anyway, I relocated my solar controller and it works perfectly now.
Hope this helps.